I mean, they become white when I can click on them, but it's a bit counter-intuitive if I have more money than the upgrade cost, but the button remains in its "idle" state. What I meant was some sort of "greyed-out" button with a description like, "you cannot buy this upgrade right now"/"requires upgrade X".
Regarding the screen, I did not think about it. I thought that the problem would've been solved with fullscreen anyway, that's why I told you about that. :)
I'm about to play your game again right now, btw.