As I said,
If the rule
#content, .entry-content h1, .entry-content h2 .side-by-side { float: left; margin-right: 2px }
In not in the Custom CSS Rule box, then you must have added
.side-by-side { float: left; margin-right: 2px }
In one of the CSS+ boxes.
You need to check all your CSS+ boxes
