Here is another small tip for designing drupal website. When you develop sites in drupal you be definitely need to modify the existing css file of the theme you are using. my suggestion is don't mess much with the original theme. Keep it as it is or change it only when you really have to.
Just create your own css file, lets say we call it MyOwn.css and write your code in it. Then edit the .info file of the theme you are using and add the following line in it.
- stylesheets[all] = style.css
- stylesheets[all] = ribbon.css
- stylesheets[all] = aggregator.css