in ThemePark, go way to the left and select your theme. At the bottom there should be 4 options... Add, Remove, something else I forget, but then very important: Options. In Options, activate MenuEnhancer.
Next, head to Global Elements > Menubars and Menus. You should see a bunch of new things to be edited, what you're looking for a Root item caps. All of the bright colors and new things to edit can be a tad intimidating, but it's really not that big of a chore.
A few things to point out... if you make something only available through MenuEnhancer transparent, the normal graphic for it will show through. So if everything already looks the way you want, and all you want to change is the Root menu item caps, then just make everything but the Root menu item caps transparent.
One more thing... if you go all gung-ho and put unique graphics into every single element made available by MenuEnhancer, all of the normal, non-menuenhancer graphics will be covered up (and essentially ignored) except for Menu Background, which is used behind label color picking in the Finder and also extends a few pixels above and below each menu.
Hope this helps, and good luck!