STSCommerce is using LessJS as a CSS Preprocessor so you can use regular CSS or Less syntax( http://lesscss.org/)
You will need to use the Inspector of your Browser to view your section HTML Structure. Right click the section, select Inspect this element. You will need to disable Builder Context menu: read here
background-color:#000; color:#fff; // Regular CSS
background-color:@bg-color;color:@text-color;// LESS Syntax
@bg-color and @text-color is a less variable loaded from theme variable section, refer here for details Edit theme Variable
Apply style for child element of the section, such as a title module
.module-title{
background-color:#000;
color:#fff;
}