I figured it out:
Only the font size can be changed.
The font size only, (and strangely enough, not the font family), gets inherited
from 'System Menu Font'.
You might see not any change happen though, until you go to Windowblinds and
apply a random skin, (just while being in Skinstudio, no need to shut it down first),
and then re-apply the skin you're working on, (thanks to DaveRI for pointing this out).
If the colors are different on another computer, it's probably because the toolbar
is transparent and the other computer has another background.
