You need to modify the style sheet. Its called sps.css. It is at in the following root on the machine where Sharepoint is installed.

C:\Program Files\Common Files\Microsoft Shared\web server extensions\60\TEMPLATE\LAYOUTS\1033\STYLES \sps.css

You need to figure out in that file, which one is responsible for the header text. Easy way to see it is, look in the view source of the page  and find out the class used for that  web  part. Then modify that class in the css file. It is not that difficult.

Good luck.