CSS theme breakdown

I wonder if there’s a good tutorial someplace that breaks down one of the default themes, pretty much line-by-line. I go into the Editor, and the lines of CSS, and the separate CSS sections, e.g. “Menu,” don’t make a lot of sense to me.

I have a very difficult time tracing back some area or line on a web page to the CSS coding that affects it. When I try to use the Inspect Element, in Firefox for instance, it doesn’t necessarilaly show me the lines of coding for it.

I just can’t trace back the element to the CSS coding affecting it.