Here's what I always do.
+ Do a rebuild, just in case.
+ Download the relevant HTML-files locally to my PC.
+ Edit the CSS-file(s) in place.
+ Fine-tune until I like the results.
+ Upload the changed CSS-file(s).
+ Do a rebuild.
+ Voila!
I have extracted all the CSS-stuff from the index files and put them in a separate file called "Blogger.css" so that I have all the style-dependant stuff localized.
Is this what you were asking?
By the way, I use TopStyle Pro which is a fantastic CSS-editor that allows you to make changes on the fly and see how that effects your site's appearance.
Hope this helps.