1) Using your FTP program, download a copy of your stylesheet to your local computer. Rename it to something (like stylesheet.old).

2) Using Firefox and Firebug, make your changes.

I don't use Firebug. I have the web developer addon. I think they're about the same.

3) right click on the stylesheet in Firebug. You'll get a little popup menu. Click on "select all". Everything in the stylesheet window will be highlighted. Press ctrl and c (letter c) on your keyboard at the same time.

4) Open a new file with either Notepad++ or Crimson Editor.

5) Paste what you copied into it (press ctrl v (the letter v)).

6) Save the file as stylesheet.css

7) upload (FTP) it to your website and overwrite the stylesheet.css that you downloaded in step 1.

Your changes should be permanent.