I found questions regarding why the default built-in startpage is not conforming to the system theme. https://support.mozilla.org/bm/questions/1286932#answer-1312559 https://support.mozilla.org/en-US/questions/1316562 The problem is, my startpage still doesn't change to darkmode. All my about:* like settings, plugin and 'blank page' however does change with the system theme. Web pages like google also reacts fine to the change. I can customize it, but I'd prefer for this to work on 'clean installs' as well :)