Add bootswatch

Is it already working in the bootstrap5:^4.0@alpha version ?

I would guess that the last three items on the list are still not done.

Is there any documentation on how to use a bootswatch theme? I don't see an option to select in the UI

As mentioned in description, work in progress.
Bootswatch was added, the rest requires help and/or financing.

It's pretty simple to do manually.

1) download (and rename? it's been a while) the main _bootswatch.scss to your /scss dir and keep the variables file handy.
2) add @import 'bootswatch'; to style.scss
3) copy over the variables from the bootswatch theme (flat, etc) into _variables_bootstrap.scss
4) add $form-text-margin-top: 0; back into the var scss (the other two changes were in my flat theme vars, you can doublecheck whatever one you're using).