I don't think its possible to save your edited CSS changes from Firebug. Certainly I've never had any joy doing this. If you make some changes in Firebug and then save the page I very much expect that you won't end up with what you'd hoped for!
Unfortunately doctoring an existing theme isn't as easy as you might hope. I think the main problem is that even if you copy the existing theme.css then any paths that are referred to from that file (images etc) wil now probabaly not work, as they'll be looking for the images as a relative path from your new file, not from where the existing theme is located.
In the nearish future I hope that you will be easily able to specify a "supplemental" CSS file for any theme, which will avoid you having to create a new theme based on an old one. If you can wait a while then I would recommend doing so! If you're really keen to get a new custom theme working based on an existing theme then you need to do something like this:
The first step is to get hold of the CSS rules for the existing theme. There are a few CSS files but most of these are the base stuff for laying out the apps etc which will be included for you anyway, so don't need to worry about these. The key file is the one called theme.css.
In actual fact, in the them you're using, theme.css file loads up two other css files - body.css and style.css. So what we're looking for as a starting point is that you create a new css file that contains the rules from body.css followed by the rules from style.css.
Any URLs referred to from within your newly created CSS file will need to be updated to explicitly load images/etc from the old theme's resources. So for example background-image: url('topBullet.gif'); will need to become background-image: url('/resources/themes/template6e/topBullet.gif'); - and the same for all others.
Once you've done that then you should find that if you upload your new CSS file into the Custom Themes section of your assets library, and then set this as the CSS fiel for the Custom Theme in the Look and Feel settings, then you should have a theme that looks like the old one but is now using your CSS instead.
So, after you've done all that, you can then edit your new CSS file to have your new colours etc and upload a new revision into the assets library.
Like I said, it's all a bit long winded. But have a go if you are feeling confident. Otherwise perhaps wait until we've implemented the change to allow you to upload your own supplemental CSS file to any theme!
Joe - Voice Admin