By now, you probably know that you can use custom properties for individual color components, to avoid repeating the same color coordinates multiple times throughout your theme. You may even know that you can use the same variable for multiple components, e.g. HSL hue and lightness:

:root {
	--primary-hs: 250 30%;
}

h1 {
	color: hsl(var(--primary-hs) 30%);
}

article {
	background: hsl(var(--primary-hs) 90%);
}

article h2 {
	background: hsl(var(--primary-hs) 40%);
	color: white;
}

Here is a very simple page designed with this technque:

Unlike preprocessor variables, you could even locally override the variable, to have blocks with a different accent color:

:root {
	--primary-hs: 250 30%;
	--secondary-hs: 190 40%;
}

article {
	background: hsl(var(--primary-hs) 90%);
}

article.alt {
	--primary-hs: var(--secondary-hs);
}

This is all fine and dandy, until dark mode comes into play. The idea of using custom properties to make it easier to adapt a theme to dark mode is not new. However, in every article I have seen, the strategy suggested is to create a bunch of custom properties, one for each color, and override them in a media query.

This is a fine approach, and you’ll likely want to do that for at least part of your colors eventually. However, even in the most disciplined of designs, not every color is a CSS variable. You often have colors declared inline, especially grays (e.g. the footer color in our example). This means that adding a dark mode is taxing enough that you may put it off for later, especially on side projects.

The trick I’m going to show you will make anyone who knows enough about color cringe (sorry Chris!) but it does help you create a dark mode that works in minutes. It won’t be great, and you should eventually tweak it to create a proper dark mode (also dark mode is not just about swapping colors) but it’s better than nothing and can serve as a base. The basic idea is to use custom properties for the lightness of colors instead of the entire color. Then, in dark mode, you override these variables with 100% - lightness. This generally produces light colors for dark colors, medium colors for medium colors, and dark colors for light colors, and still allows you to define colors inline, instead of forcing you to use a variable for every single color. This is what the code would look like for our example:

root {
	--primary-hs: 250 30%;
	--secondary-hs: 190 40%;

	--l-0: 0%;
	--l-30: 30%;
	--l-40: 40%;
	--l-50: 50%;
	--l-90: 90%;
	--l-100: 100%;
}

@media (prefers-color-scheme: dark) {
	:root {
		--l-0: 100%;
		--l-30: 70%;
		--l-40: 60%;
		--l-90: 10%;
		--l-100: 0%;
	}
}


body {
	background: hsl(0 0% var(--l-100));
	color: hsl(0 0% var(--l-0));
}

h1 {
	color: hsl(var(--primary-hs) var(--l-30));
}

article {
	background: hsl(var(--primary-hs) var(--l-90));
}

article h2 {
	background: hsl(var(--primary-hs) 40%);
	color: white;
}

footer {
	color: hsl(0 0% var(--l-40));
}

The result looks like this in light & dark mode:

Note that here we indiscriminately replaced all lightnesses with lightness variables. In reality, we don’t need to be quite as sweeping. For example, the article titles would actually look better and would have better contrast if we just kept them the same:

These are decisions that are easy to make while you go through your CSS replacing lightness percentages with variables and previewing the result.

The problem with HSL

But why were the article headers easier to read with their original colors than with inverted lightness? The root cause is that HSL lightness does not actually correspond to what humans perceive as lightness, and the same lightness difference can produce vastly different perceptual differences.

That is the big problem with this approach: it assumes that HSL lightness actually means something, but as we’ve discussed before, it does not. Yellow and blue have the same HSL lightness (50%) for crying out loud! Also, you will notice that your dark colors have smaller differences between them than your light colors, because HSL is not perceptually uniform.

Does that mean the technique is not useful for anything other than a placeholder while we develop our real dark mode, if that?

Well, things are not quite as grim.

Soon enough, we will get LCH colors in the browser. The first browser implementation just recently shipped in Safari and there is activity in that space among the other browser vendors too.

LCH is a much better color space for this technique, because its lightness actually means something, not just across different lightnesses of the same color, but across different hues and chromas.

This next example needs Safari TP 120+ . Compare these two gradients, the top one showing various HSL colors all with lightness 50%, and the bottom various LCH colors, all with lightness 50%. You can even adjust the slider and try different lightnesses:

Here is a screenshot for those of you who don’t have access to Safari TP 120+:

Notice that in HSL, some colors (like yellow and cyan) are much lighter than others. In LCH, all colors at the same lightness are, well, the same lightness.

Keep in mind that LCH chroma doesn’t really correspond to HSL lightness, so even though we’ve set it to the same number, it doesn’t correspond to the same thing.

So, how would this technique work with LCH colors? Let’s try it out!

I used this tool to convert the existing HSL colors to LCH, then tweaked the values manually a bit as the initially converted colors didn’t look nice across all LCH lightnesses (note that HSL colors with the same hue and saturation may have different hue and chromas in LCH. The opposite would defeat the point!). This is what this technique looks like with LCH colors instead (you will need Safari TP 120 or later to view this):

And here is a screenshot:

Not only does dark mode look a lot better, but even in light mode, our two alternate colors actually look more uniform since they have the same LCH lightness.

Here is a comparison of the two dark modes:

Comparison of the two auto-generated dark modes, via HSL lightness on the left and LCH lightness on the right.

Here you can see an animated comparison of them over each other:

Note that in reality, until LCH colors are reliably supported everywhere you’d need to provide a fallback via @supports, but for brevity, I did not include one in this demo.

Automating generation of lightness variables

If you are using a preprocessor that supports loops, such as Sass, you can automate the generation of these variables, and make them even more granular, e.g. every 5%:

:root {
    @for $i from 0 through 20 {
        --l-#{$i * 5}: #{$i * 5}%;
    }
}

@media (prefers-color-scheme: dark) {
    :root {
        @for $i from 0 through 20 {
            --l-#{$i * 5}: #{100 - $i * 5}%;
        }
    }
}

Can we make lightness variables more DRY?

Some of you may have disliked the repetition of values: we need to declare e.g. --l-40 as 40%, then set it to 60% in dark mode. Can’t we derive it somehow, by subtracting the value we already have from 100%?

Those with experience in programming may try something like this:

--l-40: calc(100% - var(--l-40));

However, this will not work. CSS is not an imperative language. It does not have steps of calculation, where variables have different values before and after each step. There is no such concept of time, all declarations that are currently applied, need to be true at once. It’s more similar to the reactive evaluation of spreadsheet formulas than to computation in JS and other popular programming languages (there are general purpose reactive programming languages, but they are less well known). Therefore, declarations like the one above are considered cycles: since --l-40 cannot refer to itself, this is an error, and --l-40 would be set to its initial value as an error recovery mechanism (since CSS cannot throw errors).

So, is there a way to avoid declaring lightness variables twice, once for light mode and once for dark mode?

There is, but I wouldn’t recommend it. It makes the code more convoluted to read and comprehend, for little benefit. But for the sake of intellectual amusement, I’m going to describe it here.

Instead of setting --l-40 to 40%, we are going to set it in terms of its difference from 50%, i.e. -10%. Then, calc(50% + var(--l-40)) gives us 40% and calc(50% - var(--l-40)) gives us 60%, the two values we need. We can therefore declare one variable that is -1 in dark mode and 1 in light mode, and just multiply with that.

Here is a subset of what our code would be like with this:

:root {
	--dm: 1;

	/* Example declaration: */
	--l-40: -10%;
}

@media (prefers-color-scheme: dark) {
	:root {
		--dm: -1;
	}
}

/* Example usage: */

footer {
	color: hsl(0 0% calc(50% + var(--dm) * var(--l-40));
	/* Ewww! */
}

And hopefully now you can see why I wouldn’t recommend this: it makes usage much more complicated, to DRY up a few declarations that would only be specified once. It’s this kind of obsessive adherence to DRY that programmers eventually realize is counterproductive.

---

Liked this article? Sign up for my Smashing Workshop on Dynamic CSS for more content like this!

By now, you probably know that you can use custom properties for individual color components, to avoid repeating the same color coordinates multiple times throughout your theme. You may even know that you can use the same variable for multiple components, e.g. HSL hue and lightness:

:root {
	--primary-hs: 250 30%;
}

h1 {
	color: hsl(var(--primary-hs) 30%);
}

article {
	background: hsl(var(--primary-hs) 90%);
}

article h2 {
	background: hsl(var(--primary-hs) 40%);
	color: white;
}

Here is a very simple page designed with this technque:

Unlike preprocessor variables, you could even locally override the variable, to have blocks with a different accent color:

:root {
	--primary-hs: 250 30%;
	--secondary-hs: 190 40%;
}

article {
	background: hsl(var(--primary-hs) 90%);
}

article.alt {
	--primary-hs: var(--secondary-hs);
}

This is all fine and dandy, until dark mode comes into play. The idea of using custom properties to make it easier to adapt a theme to dark mode is not new. However, in every article I have seen, the strategy suggested is to create a bunch of custom properties, one for each color, and override them in a media query.

This is a fine approach, and you’ll likely want to do that for at least part of your colors eventually. However, even in the most disciplined of designs, not every color is a CSS variable. You often have colors declared inline, especially grays (e.g. the footer color in our example). This means that adding a dark mode is taxing enough that you may put it off for later, especially on side projects.

The trick I’m going to show you will make anyone who knows enough about color cringe (sorry Chris!) but it does help you create a dark mode that works in minutes. It won’t be great, and you should eventually tweak it to create a proper dark mode (also dark mode is not just about swapping colors) but it’s better than nothing and can serve as a base. The basic idea is to use custom properties for the lightness of colors instead of the entire color. Then, in dark mode, you override these variables with 100% - lightness. This generally produces light colors for dark colors, medium colors for medium colors, and dark colors for light colors, and still allows you to define colors inline, instead of forcing you to use a variable for every single color. This is what the code would look like for our example:

root {
	--primary-hs: 250 30%;
	--secondary-hs: 190 40%;

	--l-0: 0%;
	--l-30: 30%;
	--l-40: 40%;
	--l-50: 50%;
	--l-90: 90%;
	--l-100: 100%;
}

@media (prefers-color-scheme: dark) {
	:root {
		--l-0: 100%;
		--l-30: 70%;
		--l-40: 60%;
		--l-90: 10%;
		--l-100: 0%;
	}
}


body {
	background: hsl(0 0% var(--l-100));
	color: hsl(0 0% var(--l-0));
}

h1 {
	color: hsl(var(--primary-hs) var(--l-30));
}

article {
	background: hsl(var(--primary-hs) var(--l-90));
}

article h2 {
	background: hsl(var(--primary-hs) 40%);
	color: white;
}

footer {
	color: hsl(0 0% var(--l-40));
}

The result looks like this in light & dark mode:

Note that here we indiscriminately replaced all lightnesses with lightness variables. In reality, we don’t need to be quite as sweeping. For example, the article titles would actually look better and would have better contrast if we just kept them the same:

These are decisions that are easy to make while you go through your CSS replacing lightness percentages with variables and previewing the result.

The problem with HSL

But why were the article headers easier to read with their original colors than with inverted lightness? The root cause is that HSL lightness does not actually correspond to what humans perceive as lightness, and the same lightness difference can produce vastly different perceptual differences.

That is the big problem with this approach: it assumes that HSL lightness actually means something, but as we’ve discussed before, it does not. Yellow and blue have the same HSL lightness (50%) for crying out loud! Also, you will notice that your dark colors have smaller differences between them than your light colors, because HSL is not perceptually uniform.

Does that mean the technique is not useful for anything other than a placeholder while we develop our real dark mode, if that?

Well, things are not quite as grim.

Soon enough, we will get LCH colors in the browser. The first browser implementation just recently shipped in Safari and there is activity in that space among the other browser vendors too.

LCH is a much better color space for this technique, because its lightness actually means something, not just across different lightnesses of the same color, but across different hues and chromas.

This next example needs Safari TP 120+ . Compare these two gradients, the top one showing various HSL colors all with lightness 50%, and the bottom various LCH colors, all with lightness 50%. You can even adjust the slider and try different lightnesses:

Here is a screenshot for those of you who don’t have access to Safari TP 120+:

Notice that in HSL, some colors (like yellow and cyan) are much lighter than others. In LCH, all colors at the same lightness are, well, the same lightness.

Keep in mind that LCH chroma doesn’t really correspond to HSL lightness, so even though we’ve set it to the same number, it doesn’t correspond to the same thing.

So, how would this technique work with LCH colors? Let’s try it out!

I used this tool to convert the existing HSL colors to LCH, then tweaked the values manually a bit as the initially converted colors didn’t look nice across all LCH lightnesses (note that HSL colors with the same hue and saturation may have different hue and chromas in LCH. The opposite would defeat the point!). This is what this technique looks like with LCH colors instead (you will need Safari TP 120 or later to view this):

And here is a screenshot:

Not only does dark mode look a lot better, but even in light mode, our two alternate colors actually look more uniform since they have the same LCH lightness.

Here is a comparison of the two dark modes:

Comparison of the two auto-generated dark modes, via HSL lightness on the left and LCH lightness on the right.

Here you can see an animated comparison of them over each other:

Note that in reality, until LCH colors are reliably supported everywhere you’d need to provide a fallback via @supports, but for brevity, I did not include one in this demo.

Automating generation of lightness variables

If you are using a preprocessor that supports loops, such as Sass, you can automate the generation of these variables, and make them even more granular, e.g. every 5%:

:root {
    @for $i from 0 through 20 {
        --l-#{$i * 5}: #{$i * 5}%;
    }
}

@media (prefers-color-scheme: dark) {
    :root {
        @for $i from 0 through 20 {
            --l-#{$i * 5}: #{100 - $i * 5}%;
        }
    }
}

Can we make lightness variables more DRY?

Some of you may have disliked the repetition of values: we need to declare e.g. --l-40 as 40%, then set it to 60% in dark mode. Can’t we derive it somehow, by subtracting the value we already have from 100%?

Those with experience in programming may try something like this:

--l-40: calc(100% - var(--l-40));

However, this will not work. CSS is not an imperative language. It does not have steps of calculation, where variables have different values before and after each step. There is no such concept of time, all declarations that are currently applied, need to be true at once. It’s more similar to the reactive evaluation of spreadsheet formulas than to computation in JS and other popular programming languages (there are general purpose reactive programming languages, but they are less well known). Therefore, declarations like the one above are considered cycles: since --l-40 cannot refer to itself, this is an error, and --l-40 would be set to its initial value as an error recovery mechanism (since CSS cannot throw errors).

So, is there a way to avoid declaring lightness variables twice, once for light mode and once for dark mode?

There is, but I wouldn’t recommend it. It makes the code more convoluted to read and comprehend, for little benefit. But for the sake of intellectual amusement, I’m going to describe it here.

Instead of setting --l-40 to 40%, we are going to set it in terms of its difference from 50%, i.e. -10%. Then, calc(50% + var(--l-40)) gives us 40% and calc(50% - var(--l-40)) gives us 60%, the two values we need. We can therefore declare one variable that is -1 in dark mode and 1 in light mode, and just multiply with that.

Here is a subset of what our code would be like with this:

:root {
	--dm: 1;

	/* Example declaration: */
	--l-40: -10%;
}

@media (prefers-color-scheme: dark) {
	:root {
		--dm: -1;
	}
}

/* Example usage: */

footer {
	color: hsl(0 0% calc(50% + var(--dm) * var(--l-40));
	/* Ewww! */
}

And hopefully now you can see why I wouldn’t recommend this: it makes usage much more complicated, to DRY up a few declarations that would only be specified once. It’s this kind of obsessive adherence to DRY that programmers eventually realize is counterproductive.

---

Liked this article? Sign up for my Smashing Workshop on Dynamic CSS for more content like this!

Reading Time: 4 minutes One of the things I’ve been doing for the past few months (on and off—more off than on TBH) is rewriting Bliss to use ESM 1. Since Bliss v1 was not using a modular architecture at all, this introduced some interesting challenges. Bliss is essentially a collection of helper functions. Most of these functions have […]

One of the things I’ve been doing for the past few months (on and off—more off than on TBH) is rewriting Bliss to use ESM 1. Since Bliss v1 was not using a modular architecture at all, this introduced some interesting challenges. Bliss is essentially a collection of helper functions. Most of these functions have […]

One of the things I’ve been doing for the past few months (on and off—more off than on TBH) is rewriting Bliss to use ESM 1. Since Bliss v1 was not using a modular architecture at all, this introduced some interesting challenges. Bliss is essentially a collection of helper functions. Most of these functions have a number of different signatures, to allow for more compact, readable code. The functions can be used for single things (one element, one set of arguments) or they can operate en masse (arrays of elements, object literals with multiple key-value pairs). As you might guess, this practice has been strongly inspired by the heavy use of overloading in jQuery, which was one of the driving factors behind its huge success.

For example, let’s take $.style(). It can be used to set a single CSS property, on a single element, being a rather thin abstraction over element.style:

$.style(element, "top", rect.top);

It can also be used to set a single CSS property on multiple elements:

$.style($$(".popup"), "top", rect.top);

It can also be used to set multiple properties on a single element:

$.style(element, {
	top: rect.top,
	right: rect.right,
	bottom: rect.bottom,
	left: rect.left
);

Or to set multiple properties on multiple elements:

$.style($$(".popup"), {
	top: rect.top,
	right: rect.right,
	bottom: rect.bottom,
	left: rect.left
});

I’m a strong believer in overloading for handling both aggregate operations, as well as singular data. Supporting only aggregate operations would mean that developers have to pointlessly wrap single values in object literals or arrays. E.g. if $.style() only accepted arrays and object literals, our first example would be:

$.style([element], {top: rect.top});

Not the end of the world, but certainly annoying and error-prone. Developers would often try setting the pair as separate arguments because it’s more natural, remember it doesn’t work, then adjust their code.

The opposite situation is much worse. If $.style() only supported singular operations, our last example would be:

let values = {
	top: rect.top,
	right: rect.right,
	bottom: rect.bottom,
	left: rect.left
};
for (let element of $$(".popup")) {
	for (let property in values) {
		$.style(element, property, values[property]);
	}
}

Yikes! You don’t need a library for that! Just using element.style and Object.assign() would have actually fared better here:

for (let element of $$(".popup")) {
	Object.assign(element.style, {
		top: rect.top,
		right: rect.right,
		bottom: rect.bottom,
		left: rect.left
	});
}

$.style() is not unique here: any Bliss function that accepts a main target element (the function’s subject as it’s called in the Bliss docs) also accepts arrays of elements. Similarly, any Bliss function that accepts key-value pairs as separate arguments, also accepts object literals with multiple of them.

In talks about API Design, I have presented this pattern (and overloading in general) as an instance of the Robustness principle in action: “Be liberal in what you accept” is good practice for designing any user interface, and APIs are no exception. An analog in GUI design would be bulk operations: imagine if e.g. you could only delete emails one by one?

In JS, overloading is typically implemented by inspecting the types and number of a function’s arguments in the function, and branching accordingly. However, doing this individually on every function would get quite repetitive. Consider the following, very simplified implementation of $.style() with the overloading logic inlined:

style(subject, ...args) {
	if (Array.isArray(subject)) {
		subject.forEach(e => style(e, ...args));
	}
	else if ($.type(args[0]) === "object" && args.length = 1) {
		for (let p in args[0]) {
			style(subject, p, args[0][p]);
		}
	}
	else {
		subject.style[args[0]] = args[1];
	}

	return subject;
}

Note that the actual code of this function is only 1 line out of the 13 lines of code it contains. The other 12 are just boilerplate for overloading. What a nightmare for maintainability and readability!

In Bliss v1, all functions were contained a single file, so they could be defined in their most singular version (one element, a single key-value pair as separate arguments etc), and the aggregate signatures could be automatically generated by looping over all defined functions and wrapping them accordingly.

However, in Bliss v2, each function is defined in its own module, as a default export. There is also a module pulling them all together and adding them on $, but people should be able to do things like:

import style from "https://v2.blissfuljs.com/src/dom/style.js";

And style() would need to support its full functionality, not be some cut down version allowing only single elements and one property-value pair. What use would that be?

This means that the overloading needs to happen in the module defining each function. It cannot happen via a loop in the index.js module. How can we do this and still keep our code maintainable, short, and easy to change? I explored several alternatives.

(We are not going to discuss the implementation of overload() in each case below, but if you’re interested in the current one, it’s on Github. Do note that just like everything in Bliss v2, it’s subject to heavy change before release)

Option 1: Inside each function

export default function style(subject, ...args) {
	return overload(subject, args, (element, property, value) => {
		element.style[property] = value;
	})
}

While this at first seems like the most natural way to abstract the inlined code we previously had, it’s the most verbose and hard to read. Furthermore, it adds extra code that needs to be executed every time the function is called and needs us to pass the current execution context through. It’s far better to go with a solution that takes the singular function as input, and gives you a modified function that just works. That’s what the next two options use.

Option 2: Wrap with overload()

export default overload(function style(element, property, value) {
	element.style[property] = value;
});

Option 3: Overload at export

function style(element, property, value) {
	element.style[property] = value;
}

export default overload(style);

Options 2 and 3 are very similar. I was originally inclined to go with 2 to avoid typing the function name twice, but I eventually concluded that it made the code harder to read, so I went with option 3: Declaring the function, then overloading it & exporting it.

I wasn’t super happy with any of these options. Something inside me protested the idea of having to include even a line of boilerplate in every single module, and almost every Bliss function depending on another module. However, in the large scheme of things, I think this boilerplate is as minimal as it gets, and certainly beats the alternatives.

Have you had to perform a transform on a number of different modules in your code? How did you abstract it away?

1 You can see the rewrite progress in the v2 branch on Github, and even use v2.blissfuljs.com to import modules from and experiment. Note that at the time of writing, all of the progress is in the code, the docs and tests are still all about v1.