Fortunately, you're wrong! If you install Firefox and its Web Developer extension, you will see in the toolbar a "CSS" button. Click that and select "Edit CSS", and you will get a display of the stylesheets for the current page on the left and the page itself on the right. Any changes you make to the stylesheets will show up immediately on the page. It is only temporary; to make the changes permanent you have to save them and FTP the file to your server.

It is best to have a local server set up, but that is not really difficult. See this article in the wiki > Customisation > More Advanced for full instructions.