Editing my theme

First off – am I using 4.3? I’m not sure. I’ve chosen a theme called Sketch, installed it, and don’t know where to go next.

I came from Cargo Collective (where editing the CSS was on the side and live) and have experience in HTML and CSS. Now, when I go to customize my theme here on WP, it sends me to a link about editing CSS.

My question is – I’m kind of in a place where I don’t know what I want to know – where is the CSS of this theme that I picked? I just want to edit that.. right?? I know there is a file that I downloaded (WP 4.3) and that gave me Dreamweaver files. So.. now I have Dreamweaver files of random CSS and I’m still not sure where to edit the CSS of the theme that I picked. Or, better yet, I want to build my theme.

I’ve watched Treehouse videos, read wordpress links, etc. Still lost.