Windows Phone 7 offers very less options in the field of user Customization. The only themes options allowed in this phone is to change the Tile Colors and it.
Many users have managed and found a way to allow us to do even more by making it possible to Change the Background Color as well as use Custom Tile Colors of our own choice.
You just need a Developer Unlocked Device for this hack.
You can follow this thread at XDA to Dev Unlock your WP7 Device. 1. Download Registry Editor and Deploy it to your WP7. Download the Registry Editor for your Windows Phone 7 from here. Next to use a Custom Background you will manually need to modify some Registry Settings. (Note this an risky option but advanced users should do it)
Use the registry editor to modify the background of the dark theme go to the Keys and change the value to your favorite color in HEX.
To modify the “Dark Theme” edit the following registry keys :
[HKLM\ControlPanel\themes\1\Basethemecolor]
[HKLM\ControlPanel\themes\1\TextHighContrast]
[HKLM\ControlPanel\themes\1\TextBackGround]
[HKLM\ControlPanel\themes\1\pageBackGroundColor]
[HKLM\ControlPanel\themes\1\SysTrayBackGroundColor]
You might also need to change the Font Color
[HKLM\ControlPanel\themes\1\TextNeutralColor]
[HKLM\ControlPanel\themes\1\ListPickerBackgroundPre ssed]
[HKLM\ControlPanel\themes\1\ContrastColor]
To modify “Light Theme” Background Edit these:
[HKLM\ControlPanel\themes\0\Basethemecolor]
[HKLM\ControlPanel\themes\0\TextHighContrast]
[HKLM\ControlPanel\themes\0\TextBackGround]
[HKLM\ControlPanel\themes\0\pageBackGroundColor]
[HKLM\ControlPanel\themes\0\SysTrayBackGroundColor]
You might also need to change the Font Color
[HKLM\ControlPanel\themes\0\TextNeutralColor]
[HKLM\ControlPanel\themes\0\ListPickerBackgroundPre ssed]
[HKLM\ControlPanel\themes\0\ContrastColor]