You will want to make sure you have selected the correct CSS elements. To do this, you can follow this guide. That guide will show you how to use your browser to help you customize CSS with Google Chrome (but the steps should be the same for other browsers).
Basically, you can use your browser to select an item that you want to customize, and you can test your customizations live. You still need to have a working knowledge of CSS, but this is the best way to do custom work.