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.