Altering Categories Box Layout

[mcpalmer]

I'm sure this has been asked many times but i can't seem to find any info on it anywhere ->

 

How do i alter the layout of the items in the categories box so that there is clear space (eg <p>) between all top level items and their children and other top level items and their children? In other words all top level and children are close together but there is a gap between other top level and their children