The most flexible way to edit the theme CSS or to add your own code and components to use SCSS. As mentioned before, there are two files already prepared for your changes - _user-variables.scss and _user.scss.
To configure the theme, e.g. to change colours for some elements, to change font sizes, etc., use _user-variables.scss. Any variable from scss/bootstrap/_variables.scss or scss/modules/_variables.scss can be overridden here with your own value.
To add your own CSS code, declare it in the _user.scss file.
To compile the SCSS, you can use the gulpfile.js included in this theme (recommended) or you can download an app for your OS and compile it that way. In case you would be using an app, don’t forget to add vendor prefixes to your code too (done by Autoprefixer in the Gulp ).
If you prefer to work with Sell as a static source and not to use Gulp or Sass, you will need to add your custom CSS through a separate CSS file. For this use, there is a blank custom.css file in the CSS folder.