The beauty of Cascading Style Sheets, or CSS, is that it makes implementing design-work onto a website super-simple. Where traditional style manipulations with HTML would lead to sifting through the code of multiple web pages to keep everything consistent, CSS makes sweeping transformations across every corner of a website.
HTML is used to handle the content (words) of a website and CSS is used to handle the design of the website (background, text color, tables)
To learn more HTML/CSS, check out these tutorials!