It's not a vandalism problem per se (in fact, we never really had a problem with intentional vandalism outside of Meta's "experiment", and very little of the well-intended-newb one either), althought the possibilities are there, of course (Fun story).
No, the deterrent is against using themes in the first place.
So far, outside of the themes we use for the alt formats (1), (2) the only articles that use CSS to redesign things outside of the main document are Please interlock our fingers again (*sigh*), We Are Slaves Of Creatures We Cant See (massively dadaistic, also tends to break a lot) and of course the Valentines 2022 Hub (as an obvious joke).
We allow for far more variance than the SCP wiki when it comes to how you format your articles. There is no need for you to additionally reshuffle the essential interface people use to navigate the site on top of that (note how all those themes were essentially re-colours of the default). Does the layout of your browser window change depending on the websites you visit? Does your ringtone change depending on what app you have currently in focus on your smartphone?
I can't ban people using themes, but I'll make it as difficult for them as possible in order to prevent it.
If you want to use some non-standard yourself, you are free to figure it out, in fact even tell others. I, however, will not be enabling such anti-design which also makes my job of administration harder (imagine we're going to introduce a toggleable dark mode for reading. Now I have X incompatible themes. Joy.)
Still, we should probably get going on the style resource one day. If you want to contribute with that, I can focus resources (= time I'd spend alphabetizing trading cards instead) on that.
EDIT Just to remind you how bad it's probably going to be, I present you my own, barely functional sandboxes with stitched-together themes: The central hub, and my out-of-date Vault canon notepad
