I downloaded hiccup's sample skin from here
but haven't been able to find the element. in that skin, it's not colored separately from the tab bar, while in older skins included with musicbee it is (such as the azure skin, which is xlmc based), so I'm wondering if this element was removed at some point. I did experiment with captionbar and tabsbar elements as well, but could only change the color of the entire bar and not the button separately.
i'm using a custom xml file with <root dependsOn="DarkVIOLET.xmlc"> in it, so maybe there is something in the original xlmc that is overwriting anything i'm doing to the menubutton element.