How to use Sass Variables?

SASS is Syntactically Awesome Stylesheet, if you are working with CSS then you should learn SASS, sass is an extension of CSS and Sass is a CSS preprocessor

How to Create and Use Mixin in the Sass?

Mixin is used in Sass, which is a CSS preprocessor. Mixin works as a function in Sass and accepts arguments. Means you can pass arguments in the mixin.