Just as you would inspect/write the code you produce in an editor like visual studio, a web developer inspects the code of a published site by using a browsers devtools. From there you can see what CSS styles are affecting which elements you inspect. Webflow has base styles for many elements which you can override. I won’t bother trying to teach a lesson on CSS in a blog post, as there are more thorough references available online. MDN is the gold standard. When it comes to implementing styles in the designer, I recommend you watch / read the tutorials in the University.