Hey mate
The background could be further - you get a shadow on an island that I guess is very far away. It makes me feel like we're in a building and that background is just a painting on the wall. Probably not the experience you wanted to achieve.
Buttons stay in the middle of the screen in full screen mode.
I like the idea of grouping buttons like you did, moving on one side and turning on the other.
I remember your reasoning behind gray buttons and that it's just a prototype but right now they look really boring.