Hi Phil,
You need to get beyond the mindset that everything boils down one to one with css class names. You also need to realize that not all possible CSS selectors are going to be pre-defined already in the css files, it is ok and expected to contsruct your own selectors.
If you want to style an h3 inside a div with class="rsstitle", it does not require another class on the h3, you can do it like this:
.rsstitle h3 { font-size: 15px;}
the selector above means style any h3 contained inside another element with the class rsstitle with the style rules inside the {}
this document is the key to mastering CSS:
http://www.mojoportal.com/css-its-all-about-understanding-selectors.aspx
Hope it helps,
Joe