CSS is cascading style sheets. It is used for designing a website. It could be defining proper structure of website or coloring of website. It is very useful code not to be used only with HTML with in CMS, MVC etc. to design any online software.
CSS can be written inside code, in a separate structure of same file or in a separate file to avoid cluttering of different kinds of code. Here is a video on how to write CSS in different ways:
If you want to learn further how to do CSS coding, here is playlist of videos available: https://www.youtube.com/watch?v=wCXws2iGTgU&list=PLGnR7Ae9qkw4Eky6m7d4cTzgcEbbTCbG-
These videos show how to work on simple code, but it is valid on all types of coding either in CMS or MVC.
Contact for tasks in CSS.