It’s very easy, all the texture files are just lying there in plain png files (inside GeonBit.UI/themes//textures) , and if you do drastic change to panels borders (for example create panel texture with a much thicker border than default) you can update the textures metadata XML file (in same folder) to make GeonBit use it properly.
Here’s an example of something awesome @matt did with it (hope it’s cool I’m showing it off Matt ;)) https://t.co/t5VR8EwfDy
Ofc he did custom shader work too to get that animated effect, but its a good example of how different UI themes can look.
PS if you edit the theme it’s recommended to rename the theme folder so next time you do nuget update the GeonBit.UI package won’t try to override your files.