Well, this question's been asked so many times that it's rather unsurprising that a solution thread's not been compiled and then stickied here... but ah well.
Either way, it's a rather complicated process. You have to download a third party application called Resource Hacker, then open up the msstyles (theme file) that you are using. If it's a Windows 7 custom theme, then it's going to be the msstyles file that you downloaded for the theme. If you're using the default Windows 7 theme, it would be Aero.msstyles.
You have to find some obscure resource in the file that I can't quite locate at the moment, modify it, save it, and reload the theme. To be honest, I think it's too much work for the outcome. But you could just use a custom theme that already has the resource modified. Look on Deviantart for themes.