Many of us store our CSS on its own page, often called something like css:theme or admin:theme and tell the site manager to look at that page for the CSS. That way you always have a history of changes and can revert to previous versions when necessary. You put the CSS in a code block starting [[code type="css"]]. In your site manager under Appearance -> Themes you then select external theme and in the box type: /css:theme/code/1 (or whatever page you called it).
All you then need to do is to change/add/delete the CSS on that page and refresh the browser to see the change.
Now the cunning part of all this is that you have can have several codeblocks on that page, each starting [[code type="css"]] and for different categories you can change the CSS. So in the site manager for a specific category you would have /css:theme/code/2, for another one /css:theme/code/3 and so on. This means you can have different designs or header images or other things on your site which are specific to categories. I often have a different header image depending on what the category is.
More information on this is at http://community.wikidot.com/howto:fiddle-with-css-the-easy-way
So when I say "in your CSS" I mean the CSS code that you have on the page you created to hold your site's CSS.