When you press the VtC button, the button's background disappears but the shadow remains:
The same appears to be happening to all buttons on the main site.
On meta sites, the buttons turn grey instead of disappearing, so the shadow does not look out of place. Maybe this is the solution to this problem?
Note: I did find a similar bug on another site, so I wouldn't be surprised if there is a problem with all buttons across the network.