If these definitions should be local in a certain course you can simply add to the course an html block in which you enter the following lines (in HTML mode)
// Styles
var strStyle="<style>";
// --- begin style definitions
strStyle+=".highlight1 {color:#000000;}";
strStyle+=".highlight2 {color:#003366;}";
strStyle+=".blinkingRedBold {color:#cc0000;font-weight:bold;text-decoration:blink;}";
// --- end style definitions
strStyle+="</style>";
document.write(strStyle);
You can add as many definitions as you want between begin and end. Then if you want to change a definition you simply edit this block, open the HTML mode and make the required changes to your definitions.
In this latter way, however, the labels will be affected by the css definitions only on the main page.
Here is one place where you can find a list of css properties. You can easily google other sources.
I hope that helps.
Itamar