You mean this

To 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
Jerry