It's available to everyone because it's in fact a small manialink hack.
With some styles, labels with a link got a focusable background that you can change with attributes focusareacolor1 (normal) and focusareacolor2 (mouse over). Each attribute takes a value of the form "rgb" or "rgba" so you have any color you want ^^.
Moreover, to make this kind of buttons freely, we're using actually 2 labels: one for the text and one the "button".
Anyway, there should be soon a new attribute bgcolorfocus for quads which will does the same thing but in a more logical way.
Edit: if you don't want to wait for updated quads, consider to go on
labels manialink to see all available styles (and so those on which focusareacolors are working)