#3013 should help wrt compiler window.
Regarding having a dark theme by default, why not, but it'll be hard to agree on which one :) (Not that I care myself, dark themes are a terrible idea that doesn't scale anyway. Troll intended :grin: )
The CSS also has no effect on the edit pane because that applies styles to lexical entities, and again no desktop themes have settings for those. So that will always have to have additional setup, hence the "colour scheme" for that.
That's not entirely true. Nowadays it's usually somewhat possible to know whether the system theme in uses is light or dark, and we could have the same info for editor themes and/or have a "dark default". So sure, people will still want to select their preferred theme, but we could have a default following a tad better the system theme in this case.