Profile Tweaks
Design and Skin Customization
From either of those two locations the fields will be saved to the users 'profile' datastore so can be used in any location where the other profile stuff is found, so if in the template you see {$_profile['profile_name']} then you can use {$_profile['profile_tweaks_css3']} to get its value back out.
If in the templates you see {$profile_name} then you can use {$profile_tweaks_css3} to get its value.
if you put {debug} then you can see what values are available to you in templates.