CSS is the language that styles websites. It controls the colors, the layout, the fonts… pretty much everything concerned with the look of the site.
Modern browser DevTools, besides being gosh-darned amazing for doing our jobs as front end web developers, can be used to strong arm our way through problems we encounter on the web through our day-to-day normal usage of it.
If you haven't dug into React yet, I'd encourage it. There are lots of very satisfying feelings that come along with it.