If you loaded a demo template, look at the path to stylesheets in the default layout. On my installation, it's mydomain.com/reset.css.
If you're using sheets (https://github.com/dirkkelly/radiant-sheets- extension) to manage stylesheets, which, I think, is baked into the release you're using, then design->stylesheets -> edit stylesheets and look at the path to the stylesheet, which should be directly underneath the "slug" field. On my installation w/sheets, the path is / css/my_stylesheet (note the absence in this case of a .css file extension). Pete