Support for Bootstrap 4

[glafir]

Will there be support for Bootstrap 4?

[manuelmeurer]

You mean Bootstrap 5, I assume? 😄

I don't personally use it yet in any of my projects, so I won't add support for it yet until I need it.
But a PR is more than welcome, and I'm happy to review and help with it, so give it a shot, @glafir!