Sure. CSS is all about applying formatting/style rules to various elements of the page. The HTML defines the elements (paragraphs, list items, headers, sections, divs, ...) and the CSS language allows us to select one or more elements and then apply rules to them.
The selector language can get complicated, but the basics are:
That's in order of descending priority.
For sure. We can use the web developer tool to play with this very element.
CSS also needs to be valid, though that's probably less important than HTML validity.
CSS layouts can be complicated, but the basics aren't too bad: Block elements stack vertically and fill their container horizontally. Inline elements flow like words in a paragraph.
The W3schools site has lots of useful CSS information