How to change color of QDockWidget float-button

How to change color of QDockWidget float-button

I can't seem to change the close and float button color in a dock widget
from back to white.
Here is what i have tried:
borderStyle.append(
"QDockWidget::close-button {color: rgb(255, 255, 255); background:
rgb(100, 100, 100) }")
The background color changes but the black 'X' and black float images don't.
Why? Anybody know how to accomplish this?
Thanks