How do i change css for a page based on whether a right column exists or not

[Ally]

I'm looking for a way to change css dynamically based on whether a right column of a sharepoint layout has content or not. Currently the column hides gracefully if its empty, and the left column will expand to fill. I have a web part that needs to expand to fill the page if the right column is empty. It would be nice to have two batches of css. One that defines the size of the web part if the right column has content and one if it doesn't. I looked into making the webpart dynamic but it is a picture slideshow with floated elements for controls and text over top and it doesn't expand gracefully without css tweaks to shift the floated elements back into alignment.If you know of any tutorials or examples that would be great.Thanks!