You mean this
It's still there I just cleared removed the color from the image.
It's in the aero.msstyle in C:\Windows\Resources\Themes\Aero & you can use Ave's Windows7 Style Builder
to change it.
If you are using the default aero theme I have WSB & can change that for you.
If you are using a custom theme you will need to ask the theme maker. It's not cool to change someone else work.
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