Here's my Stylish style for fixing the dark StatusPanel (the mouse over pop up at the bottom of the Firefox browser window).
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
.statuspanel-label {
background: #666666 !important;
color: #DDDDDD !important;
}
Create a new style, name it what you want, and paste in the above.
I will flesh this out more later.