Here's a simple example to get you started:

Suppose your CSS has the following directive:

body { font-size: 13px; }

So the base font size throughout your site is 13 pixels. Now you decide that you want some of your HTML instances to have larger text, for additional impact. You can add the following to your CSS:

.bightml { font-size: 16px; }

Now, you just add the "bightml" custom CSS class to any of the HTML features that should be given the larger font.

The presence of that custom CSS class gives you almost limitless styling possibilities for your site.

Jamie