mastodon/app/javascript/styles/mastodon/theme/index.scss

@use 'base';
@use 'dark';
@use 'light';
@use 'utils';

html {
  @include base.palette;

  &:where([data-user-theme='system']) {
    color-scheme: dark light;

    @media (prefers-color-scheme: dark) {
      @include dark.tokens;
      @include utils.invert-on-dark;

      @media (prefers-contrast: more) {
        @include dark.contrast-overrides;
      }
    }

    @media (prefers-color-scheme: light) {
      @include light.tokens;
      @include utils.invert-on-light;

      @media (prefers-contrast: more) {
        @include light.contrast-overrides;
      }
    }
  }
}

.theme-dark,
html:where(
  :not([data-user-theme='mastodon-light'], [data-user-theme='system'])
) {
  color-scheme: dark;

  @include dark.tokens;
  @include utils.invert-on-dark;
}

html[data-user-theme='contrast'],
html[data-user-theme='contrast'] .theme-dark {
  @include dark.contrast-overrides;
}

.theme-light,
html:where([data-user-theme='mastodon-light']) {
  color-scheme: light;

  @include light.tokens;
  @include utils.invert-on-light;
}