CSS is NOT a programming language, as such it does not have "commands", "or "functions" or even "pre defined variables" (or variables of any kind really). Once you put a "." ( for class) or a "#" and no spaces before any word.. it becomes a class or ID; the name space is pretty much up to you, as long as you stick to the characters in Paul's link you are OK. remeber just because you CAN do something doesnt mean you SHOULD. "div.div" or ".h1 h1" are perfectly legal, but its bound to mess with your eyes and your mind when debugging.
It is GOOD practice to make those names meaningful. YOU could name something ".leftRed"... but it could come back to bite you months later when you are trying to alter your code and wonder WHY something is "leftRed".. but if you can explain why.. then by all means.
You have to keep in mind, since it seems you are working with prefabricated code that using names that have been used in the original Style Sheet will interfere and CAN override the previous code. Some times this si what you ant , especially if you know your way around CSS AND that specific Stylke template. If you don't it can be disastrous time consuming to debug.