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It's in the aero.msstyles in C:\Windows\Resources\Themes\Aero\ I use
Ave's Windows7 Style Builder and it's a Fill Color at Explorer & Shell > Explorer > Control Panel > ControlPanel > ContentPane
If you are using the default aero theme I can change that for you and post it here.
If you are using a custom theme you will need to ask the theme maker. It's not cool to change someone else work.
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You will need to do 1 of these 2 ways of Enabling custom themes for it to work.
Enabling Custom Themes - Windows 7
Or
Theme - How to Apply Custom Themes
Jerry