Only thing I forgot to mention, and I think other instance owners will agree..
Theming and branding should be for what the instance owners wanna make, and not limited to the maximum extent of freedom.
Even if it looks as crazy as Windows 95-2000 Classic Theme Era , Also that it's probably better for a longer extent of editing, since Fluxer is AGPL-3 licensed, it gives a better way for users to make it work instead of hardcoding it into the source.
Right now, going that far basically means forking the whole thing. A config-level option (theme file, env vars, whatever) would let owners get full creative control without needing to maintain a fork just for cosmetic stuff.
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Comment by @ghost