Themes use CSS, so this is a really tough question. You can apply themes so
they DON'T use CSS, but causes extra code to be written to the pages, so I
wouldn't recommend it. In fact, in FrontPage 2003 all themes are applied
using CSS by default.
So to answer you intial question, CSS means "Cascading Style Sheets" and
what it means is that styles cascade down and are applied to elements on the
page. In a nutshell, the first listed CSS file gets applied, then the next
(any identical selectors overwrite the one before), then page level (again,
any identical selectors overwrite the ones before), and then inline.
With Themes, FrontPage generates a CSS file and by design this is always the
final external CSS link on a page.
I would say the advantage to using Themes in FrontPage is that FrontPage
then generates the CSS file for you and you don't need to learn CSS syntax.
You can also modify the theme, thereby modifying the CSS file using GUI
instead of hand-coding.
Hope that helps.
-John Jansen