Forum is not design for dark mode or dark theme and if you use third party extensions or tools to display it in dark mode or dark theme, there are things incompatible with the forum visually display.

Signature like you said and felt it is not good.
Another thing is texts in colors and in highlights with glow block. You can try to highlight some words in your post and see how they will be displayed in a dark mode. It's very annoying, hard to read and it's not good visually too.