CSS lets you design web pages with text styling, layouts, and animation. If the HTML is the stage, then CSS is the costumes and stage design.
Use a style sheet for best practice, however you can also add CSS inline.
As a color-blinded person, I’m not sure styling websites is my thing, but it sure is cool. I’m interested in seeing how large style sheets are organized, how companies manage their design styling guidelines and distribute the guidelines to developers.