There is also:
- UiKit - https://getuikit.com
- Bulma - https://bulma.io/
- Sematic - https://semantic-ui.com/
NOTE: I'm not a front end person, have used a couple of these for various personal projects.
https://react.semantic-ui.com/
They're really a joy to work with. My one beef with Semantic is that it could be easier to make themes. I know that someone is currently working on a theme generator site like Bootstrap and hopefully that will be a nice step forward.
But then I tested their examples on my mobile phone and it looked really bad. It just seemed that it wasn't responsive at all, which really surprised me.