Here is an example: I want to have 2 different classes. One for rounded corners and one for normal corners. Let’s give them an ID of bg_round and bg_normal. How would I change the CSS ID dynamically using radio buttons or a drop-down box?

I guess I want to change the class of some element (rather than chaning ID of CSS class itself of which I fail to see the point).
In this case it's a javascript question - take a look at this link - you can do it either with plain JS or in a nicer way via jQuery

Thank you, I think I've got it now.