Theming comes with 2 major features.
Select the default colors and link those colors with the default area of your website (i.e. primary, secondary and others).
The second feature is the ability to override bootstrap SAAS variables. Assuming you know SAAS and want to override the default bootstrap variables. You can easily do so by modifying the variables in the backend itself. Default SAAS variables can be found in the file JOOMLROOT/templates/astroid_template_zero/scss/bootstrap/_variables.scss