3
open the php files of the pages/forms that you want to change, and look for the class and id definitions that are used.
For instance, in register.php the most common class used is; class=head
so, in your style sheet look for this line;
.head { background-color: #111111; padding: 3px;}
and thats how it works.
You can add different class's or id's to each section that you'd like to change, just make sure that you define them in the style sheet.
carnuke has a FAQ that explains a good way to find out exactly what class's and id's are being used for each page here;
https://xoops.org/modules/smartfaq/faq.php?faqid=178
Happy xoopsing!