Are you referring to the CSS provided with a template?
Changing any css can be a little tricky, as rules written within a css file are assigned class names. Unless the webpage referring to the css use same class names, the css will not work. This means, you will also have to edit the webpage code to reflect the same css class names.
Editing an existing css file however, is a little easier. Simply open the css in your favorite text editor, and edit things like color/font/borders etc. As noted above, DO NOT change class names, unless you also change the same classnames within your webpage.