It was actually listed as one of the modifiers and as my color choices were... Not great initially, I decided to just go with the ones suggested. You might notice some other games that also use this palette. I've already stumbled upon 2, I think.
The screen fill issue is almost certainly my fault. I messed around with the window and never fixed it as it worked for me fine during development with KDE. Definitely something I will keep in mind and not do in the future. I could make a new build that fixes the window, but I'm not sure if I should. Out of curiosity, I would love to see how it looks for people so I can determine which thing I did breaks it.
And the controls... The driftiness was kinda intended, as it felt like a running speed up/slow down mechanic, but I can see how it could feel annoying. The player bumping on flat terrain is definitely not intended though. I'm relatively new to Godot and couldn't figure out how to make physics behave properly without breaking everything. Definitely open to advice for the future, but obviously not something that can be fixed for now, considering the rules.