Custom CSS documentation

Hi Guys, I am using a child theme and trying to make a fairly simple change to the theme in which there is some blank space either side of the feature image slider and header image (potentially adding this to header and footer in the future, depending on how it looks after). Can someone advise on he custom css needed for this? Also, can you please advise where I can get a schema/documentation guide that contains descriptions of the css used within the theme without needing to go into its source? Thanks. Mike