Hey! Really sorry about that - this is a bug that happens on some Linux distros, and after a lot of testing I still haven't been able to figure out why...
The bug has something to do with the Ambient Occlusion effect, so one option is to disable that in the settings menu. The other option is to run the Steam version of the game on Linux (through Steam Play) - if you send me an email at colestia3@gmail.com, I'll send through a Steam key for you to try.
Otherwise I'm happy to organise a refund. Just let me know :)
Sorry again for the issue!