This page is about developing dark mode for GitLab. For more information on how to enable dark mode, see Change the syntax highlighting theme.

How dark mode works

Short version: Reverse the color palette and override a few Bootstrap variables.

Note the following:

  • The dark mode palette is defined in app/assets/stylesheets/themes/_dark.scss. This is loaded before application.scss to generate application_dark.css
    • We define two types of variables in _dark.scss:
      • SCSS variables are used in framework, components, and utility classes.
      • CSS variables are used for any colors within the app/assets/stylesheets/page_bundles directory.
  • app/views/layouts/_head.html.haml then loads application or application_dark based on the user’s theme preference.

As we do not want to generate separate _dark.css variants of every page_bundle file, we use CSS variables with SCSS variables as fallbacks. This is because when we generate the page_bundles CSS, we get the variable values from _variables.scss, so any SCSS variables have light mode values.

As the CSS variables defined in _dark.scss are available in the browser, they have the correct colors for dark mode.

color: var(--gray-500, $gray-500);

Utility classes

We generate a separate utilities_dark.css file for utility classes containing the inverted values. So a class such as gl-text-white specifies a text color of #333 in dark mode. This means you do not have to add multiple classes every time you want to add a color.

Currently, we cannot set up a utility class only in dark mode. We hope to address that issue soon.

Using different values in light and dark mode

In most cases, we can use the same values for light and dark mode. If that is not possible, you can add an override using the .gl-dark class that dark mode adds to body:

color: $gray-700;
.gl-dark & {
  color: var(--gray-500);
Avoid using a different value for the SCSS fallback
// avoid where possible
// --gray-500 (#999) in dark mode
// $gray-700 (#525252) in light mode
color: var(--gray-500, $gray-700);

We plan to add the CSS variables to light mode. When that happens, different values for the SCSS fallback will no longer work.

When to use SCSS variables

There are a few things we do in SCSS that we cannot (easily) do with CSS, such as the following functions:

  • lighten
  • darken
  • color-yiq (color contrast)

If those are needed then SCSS variables should be used.