SASS is CSS for programmers. It gives you the building blocks that you’re used to, such as variables, conditions, and loops. It also helps you organize. CSS will improve the UI of your Shiny apps, and SASS will help you generate CSS.
Shiny is a powerful tool, but with CSS you can make it a beautiful one as well. I will show you how to add CSS style to your R Shiny project. The tl;dr version is that you should add style sheets to your www directory to leverage CSS capabilities.