Just to expand on what mary has suggested - one step before the selector itself in the menus (Settings>Site administration>Appearance>Themes) you will see a list of theme names - this is NOT the selector, but is the list of themes with their own custom settings pages. this is where you can see what settings each theme has.
But so long as a theme has a customcss box on that page - choose one that has a layout you like and then you will be able to alter the css - using firebug or similar tools to help discover the selectors that are already in place and making changes in the customcss box. Some themes will have more settings too - but the main thing would be to choose one with a layout you like as this is harder to alter without access to the server files.
Don't forget, if you are making changes to the css, turn Theme Designer Mode on ( Settings>Site administration>Appearance>Themes>Theme settings ) to see the changes you make.