There are many aspects of CSS to cover. However, don't fret! You basically learn about embedded CSS and external style sheets. The former is excellent for a site that has only 1-2 total web pages. On the other hand, the latter is good for recycled/reoccurring CSS rule sets used across a web site with more than 5-10 pages. Two different approaches to CSS exist and it is up to you to decide which approach to take so that your web page isn't just black text on a white background - or, namely bare HTML.
CSS brings a little color to your web pages. CSS is the basic technique for achieving this. There might be static site generators, but at their core they still use CSS with a bunch of extra stuff (probably imported fonts). It's also important to type the syntax correctly for CSS more than HTML.
For the mistakes I've made in this chapter, I feel HTML syntax errors tend to be more forgiving and still render the pages. However, CSS errors tend to be subtly pernicious - either they cause something to silently fail and their effects only start to become more apparently when the site starts adding many more CSS rules; or elements will just not render. I felt that it was a bit harder to debug CSS syntactical errors than for those in HTML.
By the way, VSCodium is great at detecting syntactical errors in HTML and CSS. (Don't use VSCode, since Microsoft builds that version with telemetry.) Use it as an automated check against syntactical typos, but don't rely on it too much. No matter how quirky the autotabbing is on Vim's "smart" tabbing, I still prefer Vim's configuration over VSCodium's attempt at autotabbing.