No need to touch any files, you can do it from the admin - just go to System->Configuration->RT RokMage Modules->ModalHeader and you should see an input for adding additional CSS.
The following users have thanked you: robss396
- STEP 1 IN MAGENTO DEVELOPMENT - DISABLE YOUR CACHE OR YOU WON'T SEE ANY CHANGES.
Use the new "secure" tab when replying for privately sharing links.
post for tips.