Well ... the thing is, if your markup is created properly, all you should need to change is the CSS.

A GREAT example of this is the CSS ZenGarden. Every style uses the same HTML, only the different stylesheets and images are loaded.

Stop trying to make me convince you and just code, play around with it. It's not something that you'll understand today or tomorrow. It takes a while to sink in and to understand every little detail.