Every new theme is extra work for the devs and they're already overworked. I don't know how Derpi's styles are defined (SASS/LESS/hand-written CSS), but if they use some template system then theoretically extra themes could be created. But it's still significant amount of extra work.
Ponibooru had some themes (I think BigMax made one), because the site's code was more or less frozen. But Derpibooru changes slightly almost daily (see that "changelog" link at the bottom?) and every time it changes, those extra themes would have to be checked. Ain't nobody got time for that. The themes would quickly stop working.
On a slightly related note: at the start of the season, a big newspost was made about moderation on starting unnecessary drama in comments was going to get stricter. Can't we just extend this to pics in general? Like, the rules already very strongly hint that if you're uploading a text-only drama image, maybe it doesn't just belong on an imageboard in the first place.
Derpibooru costs over $25 a day to operate - help support us financially!