probably a stupid question

Question

Why do we need to add curly brackets {} when using border-style?

Answer 1

They are like the opening and closing arrows for html tags. They "close" statements so that they don't get confused with other CSS statements.

Answer 2

I think you did a little bit missunderstand the lesson. The curly brackets around the border-style property are nothing special; it's just the same syntax as anywhere in css:
selector {property: value;}