Look at your base css file. You will see several instances like the following. These are the existing customize option that you already see in your admin panel. Section 1 will correspond with what you see in Section 1 in the admin panel. You will notice by looking through them that each one has a unique key. If you create a new section, you can name it Section 1.1 etc. From looking you will easily figure out how it works, but don't go crazy with it. I've read at some point that there are only so many that you can have. It's basically just for images, and colors. Just do a ctrl+F, and search of instances of Section, and it will highlight all of them so you can study them to see the different ones. You can download one of my themes to see how I've added sections for customization.
background-color:#263238;/** OW_Control type:color, section:1. General Settings, key:bodyColor, label:1. Page background color **/