This is a bit weird, and before I dive into this further, maybe somebody can confirm this. (I checked this with a few skins)
I have set <element id="Panel.HeaderButtonOverride.Highlight" bg="2,254,6" />
But this doesn't have any effect at all when I click, or hoover over a self-created toolbar button (text button, no icon).
I also have created a toolbar button: "View: Toggle Show Thumbnail Browser" (with icon)
As soon as I click that button, suddenly for the other 'text' toolbar button the above highlight element coloring is activated.
To illustrate,
When I press the toolbar button (1) for 'show thumbnail browser', then toolbar button (2) gets the highlight color. (But the other button (3) doesn't)

edit:
And after the MB window loses focus after that, the button gets to look like this:
