Can the browser load the CSS file? Just try to type the URL for the CSS file in the browser location bar. If not, the URL is maybe wrong. Do you get any File Not Found (404) errors in your web server log?

Where did you put the CSS file? It has to be in the web server document root, which is probably not where you keep your HTML templates. Try to put it in the same folder as one of your images and use the same URL path as for that image.