CSS is the design language of the web. You need to know the basics before we can move to advanced concepts like SCSS.
The fasted way to get started with CSS would be w3schools
Bootstrap makes it ridiculously simple for people not so keen on design to make something that's at least usable. It's really crucial to get acquainted with the basic classes for structure, forms and navigation that bootstrap offers.
Bootstrap Basic that everyone must know:
UI/UX Specialists have to go much deeper into this and will be expected to make experiences much superior to what bootstrap can offer.
Please read this document for a thorough introduction to CSS.