I have never used a pi or any sbc with a de, so maybe I'm missing something, but what stopped anyone from installing a dark theme?

woo, yay, I installed the new bookworm release on my Pi 4, and by default the desktop response was so slow that I restored back to buster, and it’s so much better again.

edit: I’m not saying to not try it, but there’s a data point if you experience the same thing. Use Win32DiskImager or something to make a backup image before so you can restore it if needed later.