This actually would be better posted in the skinning forum - but try this:

1) Depending on whether you use FF or IE, there are dev add ins that can ID the object and provide all the settings applied.  IE's is horrible in this regard, so I try to use FF whereever possible for style info (FF has Dev toolbar and CSS Viewer).  Get those and ensure the class/id

2) Check the class existence - this is more than likely in the default.css (if it is head used then it is).  I believe the cascade is default->skin->containr->portal.  This shoud be referenced in the skinning pdf in the docs.  This means you'll have to copy the class into your skin.css and override the default.css's settings.  Anything set in default will need to be overridden (from my experience - I'm still learning as well).  You could change the default.css - as there is the method to do this via the Site Settings, but any page not using your skin might look odd.  If you are not adding skins and just want to change it, that is the place to do so and then backup the file somewhere. 

3) avoid creating new styles if possible for existing items, since you will have to go back and recode as well as potentially change a number of ascx files (if you want consistency).  Rather, create a new skin based on one you like, modify it, and override the existing classes. 

Hope this helps - check out the skinning forum for many answers to this and other similar questions.