You could use Firefox (Iceweasel if you use Debian Linux) with Web Developer Toolbar, it has a CSS editor and you can apply and see your changes in real time inside the browser.
It is under CSS -> Edit CSS, the browser will open a panel containing all loaded CSS files for the page you are browsing, edit your css rules see your changes, you can save it to a file or better select all your css file (the one you'd edited), copy and paste into your original CSS file.
Be careful after editing your CSS, if you have multiple tabs and you change your tab all your rules will be gone, because it will load the CSS files related to the new tab. To prevent this massive loss, you must tell CSS Editor to "stick" with the files you'd edited, you can do that clicking on the pin icon, it's the one before the textbox for search.
There is a very handy web development tool, it's an IDE, Quanta Plus, it's for linux, it's a very useful tool for editing your files (CSS, HTML, PHP, etc), it has sintax highlight, you can manage projects, etc, and you can open your files there for easy copy-paste from Firefox.
If you are interested in Quanta Plus you can read its features.