Addresses some problems from merging #29545 without formal approval. Sorry for that y'all!
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Update mentions of Bootstrap 4 where appropriate -
Clarify that new tints and shades are available as variables -
Fix incorrect function names for tint/shade -
See also #30575 for a slightly more complete pass at CSS vars docs.
Will do my own review again, but feel free to tack things on to my list above or in the comments.