under preferences add a menu item "use dark theme" which can be checked, if you uncheck the "allow subreddits to show me custom themes".

maybe even a little "use custom css"-interface where you can play with color/font settings and it directly displays them in a demo. save them in a cookie and add a little script checking for the values. if a value in this cookie throws an error, catch it and replace that value with a default one.

(not everyone knows how to manually edit css and how to add a script that overrides the css of the site ;))

add an additional sort option, "sort by activity". pretty much the (old) standard sorting of forums, where a post bumps the thread to the top. this is very useful for small subs. (this one really bugged me about reddit...)

add some varying logos like the chans or pouet do. start a pinned thread, where people can submit their design. just randomly pick one on load - maybe add an option in preferences "always load dis logo", but not necessary. the current one is ugly af (personal opinion ;p)