Sorry, this forum does not have a dark theme, so setting Firefox's preference doesn't have any effect (Web Appearance depends on sites having color scheme style sheets). I'm sure it is on the "to do" list, but... Add-ons are blocked from injecting scripts into certain Mozilla sites that have elevated access to private data in the browser (such as this one, the Add-ons site, Firefox Accounts, etc.). If you plan to use the site a lot, it might be worth using an unofficial, community-supported method of injecting your own style rules. As a quick and dirty example (well, a 10 minute example), take a look at this thread: https://support.mozilla.org/questions/1391430