I really like the dark mode, and use it almost exclusively when surfing.
Since I mostly surf SPF from a laptop with 1920x1080 resolution, I am always sensitive to screen space. As much as beautiful art is pretty and attractive for a new visitor, it is not very useful for an old hand on his zillionth visit. So if there was an option for a most-streamlined-décor leaving all the space to content proper, it would be very welcome here.
That's for graphics.
Talking about "content proper", this site has a lot of very valuable content that make it the best encyclopaedia for unbuilt project. But it has also a lot of chaff that is the result of humans being behaving like, well, simply humans and too often kids on a "social media" not-so-proper behavior. Rather than like researchers, historians, or simply people writing text worth keeping in an encyclopedia.
There is a regular cleanup by the mods for off-topic, political and obsolete content. It is essential to preserve the signal-to-noise ratio. IMO it should be expanded to purge, or move to a different section, the content that is not "encyclopaedia worth keeping" level.
For ex, the zillion bouts of arguments between Stargazer and Hesham about speculative project without evidence are very useful for maintaining quality, but not so much a worthwhile contribution after the case has been straightened out. The threads could be cleaned up after a while.
Same for all the smart-ass, off-color and profane "wisecracks" and jokes. I'd advocate not necessarily censoring them right away to avoid emotions, but definitely removing them after a few month.
As it is, the signal-to-noise ratio on SPF is one of the best for an internet forum, but is still very far (very very very) away from encyclopaedia level worth-preserving text.
Just imagine finding puns as crass as "AUKUS - Aux culs" unfiltered on Britannica, Larousse or Wikipedia. Simply unthinkable...
Also, compare the consistent quality level of the best posters like (of the top of my head) Justo, Apophenia, PhR and all the other good contributors, versus the inconsistent level of others who regularly spew out crass and lewd "jokes" in the middle of their otherwise valuable contributions. If the crassyest and lewdest lapses were cleaned up after a while, the site level would go up immensely.
IMO this would be a much more important improvement than graphics or color themes.