Broken Titlebar Buttons in Gnome 3

[ExpatPaul]

Last night XBMC crashed (froze completely) on me and I ended up forcing my laptop to reboot in order to recover. I now notice that the titlebar buttons look like this:



I am assuming that I have broken the close graphic somehow, but have been unable to find any way of reinstalling it. Does anyone know what I need to do to fix his?

Thanks