Drupal stores a color scheme preference for each user account.
Before
Core had no color scheme preference. The default admin theme had its own dark mode setting.
After
The user entity has a new color_scheme base field. The values are auto, light and dark. The default is auto, which follows the operating system setting. Users pick a value on their profile form, under "User settings".
AccountInterface::getColorScheme() returns the value for the current user.
ThemePreprocess::preprocessHtml() adds two things:
data-color-scheme on the <html> element,
a color_scheme variable for html.html.twig.
HTML pages vary by the new user.color_scheme cache context.
The dark mode setting in the default_admin theme was removed.
Themes can use the attribute or the variable to apply the preference. This change adds no theme CSS, so no theme applies the preference yet.
user_update_11101() installs the field on existing sites.
