The Future of CSS: Easy Light-Dark Mode Color Switching with light-dark()

~

# Responding to Light or Dark Mode

To change a color value – or any other value for that matter – based on Light Mode or Dark Mode being used, you’d typically use a prefers-color-scheme Media Query to change the value of a Custom Property:

:root {
  --text-color: #333; /* Value for Light Mode */
}

@media (prefers-color-scheme: dark) {
  :root {
    --text-color: #ccc; /* Value for Dark Mode */
  }
}

When implementing Dark Mode, you typically end up with a bunch of duplicated CSS variables that set the values for each mode. The rest of your CSS then uses these custom properties for the actual declarations.

body {
  color: var(--text-color);
}

~

# Responding to Light or Dark Mode with light-dark()

A new addition to the CSS Color Module Level 5 Specification is the light-dark() function. The function accepts two color values as its arguments. Based on which color scheme you are actively using, it will output the first or the second color argument.

light-dark(<color>, <color>);

As per spec:

This function computes to the computed value of the first color, if the used color scheme is light or unknown, or to the computed value of the second color, if the used color scheme is dark.

The used color scheme is not only based on the user’s Light/Dark Mode setting, but also on the value of the color-scheme property. This similar to how System Colors get computed.

The color-scheme property allows an element to indicate which color schemes it is designed to be rendered with. These values are negotiated with the user’s preferences, resulting in a used color scheme […].

That means, for light-dark() to work, you must also include a color-scheme declaration.

:root {
  color-scheme: light dark;
}

:root {
  --text-color: light-dark(#333, #ccc); /* In Light Mode = return 1st value. In Dark Mode = return 2nd value. */
}

Because color-scheme is taken into account, that also means that you can override its value per element, to force it into a certain mode:

.dark {
  color-scheme: dark; /* light-dark() on this element and its children will always return dark */
}

~

# What about other non-<color> values and responding to other color schemes?

A common type of feedback on light-dark() I get is that it is fairly limited in what it can do: it can only do light/dark and only works with <color> values. That’s correct and is also very much intentional, because it is an intermediary step towards a final solution.

As proposed in the CSS Working Group issue, the end goal is to have a function (tentatively) named schemed-value() in the future. That function can:

• Respond to any value of color-scheme.
• Return more than <color> values

It could look something like this:

:root {
  color-scheme: dark light custom;
}

body {
  color: schemed-value(light hotpink, dark lime, custom rebeccapurple);
}

But, for now, we “only” have light-dark() and I personally think that’s fine, as it rhymes with today’s reality of what browsers can do:

• It only supports light or dark because browsers right now don’t support <custom-ident> in color-scheme, so there’s no use in supporting other values right now.
• It can only do <color> values because the parser needs to know the value type of what it is parsing ahead of time. light-dark() is explicitly defined to be a <color>.

Narrowing things down in feature scope – from the very broad schemed-value() to the slimmed down light-dark() – allowed the function as it is to be defined today, instead of putting the whole thing on the long track.

The name and syntax of light-dark() is very memorable, easy to use, and – most importantly – offers a solution to a common use-case authors are having today.

light-dark(<color>, <color>); = schemed-value(light <color>, dark <color>);

~

# Browser Support

Here is an up-to-date list of browser support for CSS light-dark():

Chromium (Blink)

❌ No support

Firefox (Gecko)

✅ Supported in Firefox 120.

Safari (WebKit)

❌ No support

The pen embedded below will indicate if the browser you are currently using supports CSS light-dark() or not:

See the Pen
CSS light-dark() Support test by Bramus (@bramus)
on CodePen.

To stay up-to-date regarding browser support, you can follow these tracking issues:

• Chromium/Blink: Issue #1490618 — Assigned (Open)
• Firefox/Gecko: Issue #1856999 — RESOLVED FIXED
• Safari/WebKit: Issue #262914 — NEW

~

# Demo

If your browser supports light-dark(), the demo below will show a few <div>s labeled .auto that respond to Light/Dark mode being toggled. The <div>s with the class .light or .dark are forced into their proper mode.

See the Pen
light-dark() Demo by Bramus (@bramus)
on CodePen.

~

# Spread the word

To help spread the contents of this post, feel free to retweet its announcement tweetpost/tootpost:

To change a color based on Light Mode or Dark Mode, you’d typically use a `prefers-color-scheme` Media Query.

To make things easier, CSS now comes with a `light-dark()` utility function.

Read https://t.co/uzcTGPo8dY to get to know the details.

Browser Support: Firefox 120. pic.twitter.com/1rmGkKy2yl

— Bramus (@bramus) October 9, 2023

~