This seems like there should be an obvious answer but I've been looking and testing for an hour.  I can't figure out how to change the OnMouseOver color of the button in the Main Menu. 

In the Menu Object Edit Screen,  I've tried changing the "background color" to no effect.   If you look at the attachment:  the button itself appears to be the "background image."  And I can't see a way to change that?  

[Again, when I go to Item for Current Page, the only option seems to be the Background Image.  If I try changing "background color" there, no change in appearance.]