update scss imports based on bootstrap 5.1 docs

[stepheneb]

trafficstars

https://getbootstrap.com/docs/5.1/customize/sass/#importing

fixes: https://github.com/twbs/bootstrap-npm-starter/issues/70

[stepheneb]

Noticed that the Toggle offcanvas button isn't working correctly. Clicking it just adds a dropshadow on the right window edge.