CSS, or Cascading Style Sheets, is a scripting language that determines
the design of a webpage. The font-style, background colour, font-size, etc.
are all determined by CSS. You can view the CSS of a website via the
Web Developer Tools (F12) by selecting the Style Editor tool.

While there you can also edit the CSS!
Alternatively, you can access the CSS file through the inspector tool as such:

To learn more about CSS, check out here.