Hi,
find Style Variable "header_background" & "header_tab_background" & "header_subtabbar_background" & "header_tabbar_background"
at admincp --> Styles & Templates--> Style Manager
and change it like you want...
Don't know but I think with this transparent header is a little problem... look into header with this style or use google: I read about it anywhere ;-)
Hope it helps.
Should be something in Documentation about it... (
http://www.vbulletin.com/docs/html/)