HTML builds the structure in which the content is placed on the page. Cascading Style Sheets take that structure and makes it beautiful. CSS is incredibly powerful for releasing your artistic energies onto a webpage. True mastery of CSS is impressive. We are not striving to currently achieve that levels of skill. What we need is to basically understand how it works so we can effectively clean-up web pages that we migrate.
Almost all of the actually styling in Drupal is done using external style sheets. This means that inside the code that we will clean up there will be very little CSS. With that said, it is important we understand how it works so we can realize how we are cleaning up the page may impact how it finally looks. As such, we are going to work through the beginning of the W3Schools CSS tutorial. It is similar in training approach to the HTML tutorial we just covered. Please start with the "CSS Introduction" and go through the "CSS Height/Width". Work through their training on their website and get "your fingers dirty" playing around with their code. Have fun with it. Explore.
This should take you about 1.5 hours.