2
The class names are called in system admin/templates
Look for the "system" files and list them. then look for system_block_login.html and click "edit"
these calls related to the style.css file in your theme folder.
In the x2t theme their is some defines for formbutton in :
/*forms elements*/
Edit: For Begginers:
The template for the x2t theme is in your extras folder in your download called x2t.tar.gz. You upload this through system admin/templates. Might give you an idea of how it is done.