Here are three themes that would make your desktop seemlesly match with firefox and iGoogle or just Google.
What you need is Firefox 3.6 and above (in order to use Firefox Personas), a personal iGoogle page, or at least a google account and some wallpapersr which can be found here, of course.
Theme number One: Lord of the Rings
Screenshots:
get the Firefox perosna One Ring to Rule Them All
Download the desktop backgrounds below.
And you can use these as Google Backgrounds too. (Change Background>From My Computer>Browse). I recommend the second wallpaper as google background.
Theme number Two: The Raindrop
Screenshots:
get the Firefox persona Raindrop Green
Theme number Three: The Ultimate Theme -Pink Sunset
Screenshots:
Firefox Persona Purple Pines Sunset
You can find the iGoogle theme here.
Click and download the wallpaper below
You can use this image as a Google background by uploading it. (Change Backgound>From My Computer>Browse)
If you are using Vista or Windows 7 or even Ubuntu 10.04 you can also change the color of the window borders and the taskbar.