Some buttons missing hover state instructions
I found two kind of buttons, that missing instructions for a hover state. Regular button and modal-dialog-linked-button. Modal-dialog-linked-buttons are in dialogs like "power off" (in that dialog buttons feels especially wierd, because you see first button marked with selection color, but when you hover cursor upon another - nothing happens, so it looks like first button is still "active"). Regular buttons hard to find, I use transmission-daemon-indicator extension to see how they looks (I'm making a theme).
With different color on hovered buttons it feels much better. Theme designer should look into it (maybe there are some tweaks needed for hovered state in _drawing.scss).