When using the "custom CSS" option in the Forms 2.0 builder, none of the custom CSS overides the CSS from the template, unless you mark it as important (using the !important; suffix at the end of all your CSS strings).
Per coding best pracitces, if you're putting up custom CSS on the form level, it should always overide the base CSS of the template. Also, per coding best practices, you should try and avoid using !important; as much as feasibly possible:
http://james.padolsey.com/usability/dont-use-important/