Skeleton is perfect.

https://github.com/dhg/Skeleton/blob/master/css/skeleton.css

It's tiny, simple, and so easy that if you read it and understand it, you could write it again yourself. It's meant to be tooled and modified to suit your needs. Bootstrap is just gigantic, it's a big unwieldily blunt instrument that eventually needs to be removed from the project.

The framework in OP bothers me because of the way it handles margins around components, makes unnecessary use of flexbox, and hides the entire sidebar of the page on mobile. Which I don't think will often be very useful.

I'm very opinionated about my css frameworks.