Editing CSS on Gallery Theme

Hello again,

One of my clients really wants the sidebar to be on the right side of the page rather than the left, and I think the Gallery theme is the best fit for his site.

I looked up how to switch it around, and got instructions to just switch the float:right and float:left in the style.css, but I can't seem to figure out where to edit this. I thought at least the child theme would have an editable css page, but alas.

Am I trying to do the impossible?