I realize you may have added
.side-by-side { float: left; margin-right: 2px }
In a CSS+ box. If that is the case, you cant do that because of the reason I juts gave you. Rules placed in aqn element CSS + box will have the Selectors of that element added to it
If you want to style your side-by-side class, place the rule in Advanced Options > Head Section > Custom CSS Rule box