Open up about:config and go to layout.css.prefers-color-scheme.content-override and set it to one of the following:

0 => Force a Dark background

1 => Force a Light background

2 => Follow the System theme (default in Firefox 94)

3 => Follow the Browser toolbar theme (default in Firefox 95+)

  • Ferk
    3·
    10 hours ago

    You don’t need about:config for this.

    This is the same setting you can configure in the preferences under “Website appearance”… if you change that you’ll see the about:config setting changes too.