Customizing the print preview toolbar color

[Falna]

I'm trying to modify the color of the print preview toolbar using userChrome.css

Now I know that this modification works on the navigation bar:

• #nav-bar {
  background-color: #f90000 !important;
  }

So why doesn't this code work?

• #print-preview-toolbar {
  background-color: #f90000 !important;
  }

I'm obviously missing something, so any clues would be appreciated...