SBS uses the popular Bootstrap framework as its CSS base and all of Bootstrap’s styles are available to you.
SBS has an in-built SCSS compiler, so all user styles are written in SCSS, although of course you can write plain CSS in an SCSS file too.
The main style file is at assets/main.scss and this is most likely the file you want to change to add your own styles. The default generated site has some styles in there already but feel free to change and override them as you please. Add new styles at the end of the file.
You can find out what class statements you need to add (S)CSS for by looking in the _layouts.system folder. See here for a description of the main layout files.