How to change column widths and resizing Related Posts

Hi there, I was wondering if there’s a way through CSS to alter the widths of the columns in this theme? For example, shortening the main column to 70% and expanding the widget sidebar to 30%.

Also, is there a way to alter the Related Posts images so you can fit four on one line (or alternatively, change it to only 3 Related Posts elements)? Thanks very much in advance!