I just dislike the stark whiteboard look of the stock program and would like to be able to change skins, or at least color schemes.
Is the reference to the window decoration (title bar and surround), menu / toolbar, page, or background (page surround) area? Please edit the question to be clearer about which area you are referring. Thanks.
Since v4.0 Firefox Personas (release notes GUI section) have been supported to provide a more colourful menu and toolbar area.