Allow theme changes

I am a dark theme users, but this dark theme is difficult to use. Would be great if we could tweak it, or write custom themes.

When viewing a list of documents, then expanding one of them, it's hard to read where the next doc starts. Different color text, or perhaps bold on the Parent (document) would make this more apparent. Alternating line colors are too different, and contrast is too low between text and background.

I'm using Linux version if that makes any difference.

Thanks