What is SASS?
quick description of what sass is
Sass is a stylesheet language that’s compiled to CSS. It allows you to use variables, nested rules, mixins, functions, and more, all with a fully CSS-compatible syntax. Sass helps keep large stylesheets well-organized and makes it easy to share design within and across projects. — Sass docs
Sass, in a nutshell, allows us to use CSS as more of a coding language, letting us use if statements and variables/nested functions It allows us to decide what colors to show dependent on the current situation given to us in the app and based on the class names around us.
We are also able to mixin and use nest rules to set global classes, so you can go ahead and write something like ul{ dark{} light{} } and get to the dark ul though .ul-dark
I'd recommend that you should use sass classes like bootstrap classes when creating your own CSS works!