The short answer is probably not.

The longer answer is that in theory it may be possible to use some kind of exploit or side-channel attack to figure out what theme is enabled or which themes are available through media queries for example with JavaScript or CSS. Tor browser uses dark mode by default, and since this is a dark theme, that doesn’t change. It only affects the chrome (UI) of the browser, not the page itself.

So that is something you should keep in mind for your threat model.

My answer is based on personal experience, some basic googling and asking the Tor community. But it still can’t hurt to do some additional research yourself. 😄

Feel free to correct me if I’m wrong.

Yep. While it seems to be just the client side theme, on Tor browser I never take a chance with modifications.

As long as it's just a client side theme that swaps out colors on device, it shouldn't be able to be used for fingerprinting, right? 

I addressed how this affects fingerprinting in this comment.