Note that you need to use MkDocs 1.0.x for this, since the Bootswatch themes aren't updated for Bootstrap 4 yet; I'll work on those updates later, but this patch should apply to both the "new" and "classic" (Bootstrap 3) versions of these themes, so I think it makes sense to do this first. Then it's less work for me when I fork this into mkdocs-bootswatch-classic. :)
Basically the same as https://github.com/mkdocs/mkdocs/pull/1935, but for these themes. I've manually tested that this works correctly on all the themes.
Note that you need to use MkDocs 1.0.x for this, since the Bootswatch themes aren't updated for Bootstrap 4 yet; I'll work on those updates later, but this patch should apply to both the "new" and "classic" (Bootstrap 3) versions of these themes, so I think it makes sense to do this first. Then it's less work for me when I fork this into
mkdocs-bootswatch-classic. :)