If you don't have it set up in a test environment (where you can make the changes and then easily FTP the files up to your hosting server) you will need to open the file (or download them from your host server, change them and re-upload them) by selecting the Theme that you are using and opening the .css file for that theme, make all your changes to your .css file and upload them. I've done it using the forums/themes/yafvision/theme.css file. I made a lot of changes to the color, layout and even made a change to the main Yaf.net logo (which is in the main default.aspx file) under
your logo.png file here
It's really pretty easy, but best done in a test environment first, if possible.