Improved dark mode default styling with the color-scheme CSS property and the corresponding meta tag

The color-scheme CSS property and the corresponding meta tag allow developers to opt their pages in to the theme-specific defaults of the user agent stylesheet.

Thomas Steiner

Background

The prefers-color-scheme user preference media feature

The prefers-color-scheme user preference media feature gives developers full control over their pages' appearances. If you are unfamiliar with it, please read my article prefers-color-scheme: Hello darkness, my old friend, where I documented everything I know about creating amazing dark mode experiences.

One puzzle piece that was only mentioned briefly in the article is the color-scheme CSS property and the corresponding meta tag of the same name. They both make your life as a developer easier by allowing you to opt your page in to theme-specific defaults of the user agent stylesheet, such as, for example, form controls, scroll bars, as well as CSS system colors. At the same time, this feature prevents browsers from applying any transformations on their own.

Browser support

prefers-color-scheme

color-scheme

The user agent stylesheet

Before I continue, let me briefly describe what a user agent stylesheet is. Most of the time, you can think of the word user agent (UA) as a fancy way to say browser. The UA stylesheet determines the default look and feel of a page. As the name suggests, a UA stylesheet is something that depends on the UA in question. You can have a look at Chrome's (and Chromium's) UA stylesheet and compare it to Firefox's or Safari's (and WebKit's). Typically, UA stylesheets agree on the majority of things. For example, they all make links blue, general text black, and background color white, but there are also important (and sometimes annoying) differences, for instance, how they style form controls.

Have a closer look at WebKit's UA stylesheet and what it does regarding dark mode. (Do a full text search for "dark" in the stylesheet.) The default provided by the stylesheet changes based on whether dark mode is on or off. To illustrate this, here is one such CSS rule using the :matches pseudo class and WebKit-internal variables like -apple-system-control-background, as well as the WebKit-internal preprocessor directive #if defined:

input,
input:matches([type="password"], [type="search"]) {
  -webkit-appearance: textfield;
  #if defined(HAVE_OS_DARK_MODE_SUPPORT) &&
      HAVE_OS_DARK_MODE_SUPPORT
    color: text;
    background-color: -apple-system-control-background;
  #else
    background-color: white;
  #endif
  /* snip */
}

You will notice some non-standard values for the color and background-color properties above. Neither text nor -apple-system-control-background are valid CSS colors. They are WebKit-internal semantic colors.

Turns out, CSS has standardized semantic system colors. They are specified in CSS Color Module Level 4. For example, Canvas (not to be confused with the <canvas> tag) is for the background of application content or documents, whereas CanvasText is for text in application content or documents. The two go together and should not be used in isolation.

UA stylesheets can use either their own proprietary or the standardized semantic system colors, to determine how HTML elements should be rendered by default. If the operating system is set to dark mode or uses a dark theme, CanvasText (or text) would be conditionally set to white, and Canvas (or -apple-system-control-background) would be set to black. The UA stylesheet then assigns the following CSS only once, and covers both light and dark mode.

/**
  Not actual UA stylesheet code.
  For illustrative purposes only.
*/
body {
  color: CanvasText;
  background-color: Canvas
}

The color-scheme CSS property

The CSS Color Adjustment Module Level 1 specification introduces a model and controls over automatic color adjustment by the user agent with the objective of handling user preferences such as dark mode, contrast adjustment, or specific desired color schemes.

The color-scheme property defined therein allows an element to indicate which color schemes it is comfortable being rendered with. These values are negotiated with the user's preferences, resulting in a chosen color scheme that affects user interface (UI) things such as the default colors of form controls and scroll bars, as well as the used values of the CSS system colors. The following values are currently supported:

• normal Indicates that the element is not aware of color schemes at all, and so the element should be rendered with the browser's default color scheme.

• [ light | dark ]+ Indicates that the element is aware of and can handle the listed color schemes, and expresses an ordered preference between them.

In this list, light represents a light color scheme, with light background colors and dark foreground colors, whereas dark represents the opposite, with dark background colors and light foreground colors.

For all elements, rendering with a color scheme should cause the colors used in all browser-provided UI for the element to match with the intent of the color scheme. Examples are scroll bars, spellcheck underlines, form controls, etc.

On the :root element, rendering with a color scheme additionally must affect the surface color of the canvas (that is, the global background color), the initial value of the color property, and the used values of the system colors, and should also affect the viewport's scroll bars.

/*
  The page supports both dark and light color schemes,
  and the page author prefers dark.
*/
:root {
  color-scheme: dark light;
}

The color-scheme meta tag

Honoring the color-scheme CSS property requires the CSS to be first downloaded (if it is referenced via <link rel="stylesheet">) and to be parsed. To aid user agents in rendering the page background with the desired color scheme immediately, a color-scheme value can also be provided in a <meta name="color-scheme"> element.

<!--
  The page supports both dark and light color schemes,
  and the page author prefers dark.
-->
<meta name="color-scheme" content="dark light">

Combining color-scheme and prefers-color-scheme

Since both the meta tag and the CSS property (if applied to the :root element) eventually result in the same behavior, I always recommend specifying the color scheme via the meta tag, so the browser can adopt to the preferred scheme faster.

While for absolute baseline pages no additional CSS rules are necessary, in the general case you should always combine color-scheme with prefers-color-scheme. For example, the proprietary WebKit CSS color -webkit-link, used by WebKit and Chrome for the classic link blue rgb(0,0,238), has an insufficient contrast ratio of 2.23:1 on a black background and fails both the WCAG AA as well as the WCAG AAA requirements.

I have opened bugs for Chrome, WebKit, and Firefox as well as a meta issue in the HTML Standard to get this fixed.

Interplay with prefers-color-scheme

The interplay of the color-scheme CSS property and the corresponding meta tag with the prefers-color-scheme user preference media feature may seem confusing at first. In fact, they play together really well. The most important thing to understand is that color-scheme exclusively determines the default appearance, whereas prefers-color-scheme determines the stylable appearance. To make this clearer, assume the following page:

<head>
  <meta name="color-scheme" content="dark light">
  <style>
    fieldset {
      background-color: gainsboro;
    }
    @media (prefers-color-scheme: dark) {
      fieldset {
        background-color: darkslategray;
      }
    }
  </style>
</head>
<body>
  <p>
    Lorem ipsum dolor sit amet, legere ancillae ne vis.
  </p>
  <form>
    <fieldset>
      <legend>Lorem ipsum</legend>
      <button type="button">Lorem ipsum</button>
    </fieldset>
  </form>
</body>

The inline CSS code on the page sets the <fieldset> element's background-color to gainsboro in the general case, and to darkslategray if the user prefers a dark color scheme according to the prefers-color-scheme user preference media feature.

Via the <meta name="color-scheme" content="dark light"> element, the page tells the browser that it supports a dark and a light theme, with a preference for a dark theme.

Depending on whether the operating system is set to dark or light mode, the whole page appears light on dark, or vice versa, based on the user agent stylesheet. There is no additional developer-provided CSS involved to change the paragraph text or the background color of the page.

Note how the <fieldset> element's background-color changes based on whether dark mode is enabled, following the rules in the developer-provided inline stylesheet on the page. It is either gainsboro or darkslategray.

The <button> element's appearance is controlled by the user agent stylesheet. Its color is set to the ButtonText system color, and its background-color and the four border-colors are set to the system color ButtonFace.

Now note how the <button> element's border-color changes. The computed value for the border-top-color and the border-bottom-color switches from rgba(0, 0, 0, 0.847) (blackish) to rgba(255, 255, 255, 0.847) (whitish), since the user agent updates ButtonFace dynamically based on the color scheme. The same applies for the <button> element's color that is set to the corresponding system color ButtonText.

Demo

You can see the effects of color-scheme applied to a large number of HTML elements in a demo on Glitch. The demo deliberately shows the WCAG AA and WCAG AAA violation with the link colors mentioned in the warning above.

Acknowledgements

The color-scheme CSS property and the corresponding meta tag were implemented by Rune Lillesveen. Rune is also a co-editor of the CSS Color Adjustment Module Level 1 specification. Hero image by Philippe Leone on Unsplash.