I just hope it's alright license-wise - for instance that a GPL 3 theme doesn't require that all Geany sources are GPL 3. But I'd say that since themes are completely independent of the rest of the Geany, loaded at the runtime in the source form, and non-essential for running Geany, they don't affect the license of Geany itself. Am I right?
I think yes but I'm not sure at all.
On the other hand in the worst case scenario that theme licenses affect Geany's license as a whole, I think we only have to deal with a single theme - the GPLv3 theme `epsilon`. Based on
https://www.gnu.org/licenses/gpl-faq.en.html#AllCompatibility
LGPLv2.1 code can be either "copied" to GPLv2 (and the result is GPLv2) or "used as a library" without any restriction - not sure which of the two things we do with themes but we should be fine.
I also searched for the difference between LGPLv2 (which most of the themes use) and LGPLv2.1 and made a diff of those and it's mostly renaming it from "GNU Library General Public License" to "GNU Lesser General Public License" and some minor wording so I think the table above is valid for LGPLv2 too as well.
So I think for now I could just move the GPLv3 theme to the ignored ones and keep the rest - I believe we should be fine.