Well I'm aware of the theme files. However, to change them you have to unpack the AppImage and either repack it or keep it unpacked, which would be OK. However, it looks like it is a scaling issue. I tried setting a theme JSON on 4.6 locally but didn't try it in the AppImage tree which now that I think about it makes sense. I will try that but I doubt it will fix my issue. But we'll see -- I'd like to be wrong.
Oh yeah -- I forgot. The AppImage mounted is RO and remounting it RW doesn't help. So I would then say the only way to do that would be to copy the AppImage tree somewhere else to do that. I'll still try it.