OS X: Fix AttentionBar close button rendering artifact.
OS X: Fix AttentionBar close button rendering artifact.

Because it's background was set to a solid color, it wasn't rendered
transparently and the gradient didn't show up underneath it.

Fixed by setting the background on Windows only.

egoitzro/poedit,dwaynebailey/poedit,PKRoma/poedit,nawawi/poedit,PatidarWeb/poedit,PKRoma/poedit,dwaynebailey/poedit,egoitzro/poedit,huoxudong125/poedit,c72578/poedit,c72578/poedit,huoxudong125/poedit,BenKeyFSI/poedit,PatidarWeb/poedit,nawawi/poedit,PKRoma/poedit,vslavik/poedit,nawawi/poedit,alexhenrie/poedit,babycaseny/poedit,PKRoma/poedit,vslavik/poedit,alexhenrie/poedit,egoitzro/poedit,vslavik/poedit,huoxudong125/poedit,alexhenrie/poedit,dwaynebailey/poedit,BenKeyFSI/poedit,dwaynebailey/poedit,PatidarWeb/poedit,babycaseny/poedit,huoxudong125/poedit,egoitzro/poedit,egoitzro/poedit,alexhenrie/poedit,huoxudong125/poedit,BenKeyFSI/poedit,nawawi/poedit,babycaseny/poedit,babycaseny/poedit,nawawi/poedit,c72578/poedit,PatidarWeb/poedit,PKRoma/poedit,babycaseny/poedit,BenKeyFSI/poedit,c72578/poedit,BenKeyFSI/poedit,PatidarWeb/poedit,dwaynebailey/poedit,vslavik/poedit,vslavik/poedit,c72578/poedit,alexhenrie/poedit