Im new to Kirby but a seasoned developer with WordPress.
Im looking forward to using Kirby as I think WP just has to much bloat.
Im used to using to my own theme with Bootstraps CSS Integrated however, I think now that its probably somewhat bloated too and I just want a simple CSS grid.
Can i simply add files into the CSS folder such as a lightweight grid stylesheet or does Kirby come with some already existing classes? Does it handle grids without the need for CSS?
Yes, you can integrate any CSS stylesheets or libraries you see fit. Kirby does not come with anything (apart from the CSS in the Starterkit), nor does it force anything on you. Templating and styling is all up to you, and that is in fact one of the strengths of Kirby.
The CSS (and JS) helpers facilitate embedding of stylesheets and scripts.
Edit: Personally, I use a modular approach with SASS, gulp and the Patterns plugin (without any library, but it would be just as easy to integrate a grid framework into such a workflow if needed).