Ugh. This is the solution:
But it didn’t make it into Beta 1.
It seems to me that without this fix the entire Bootstrap color system must be imported into Sage’s _variables.scss in order to override a single color, like the primary color. See extended conversation here:
This is profoundly annoying, but with @ben’s ‘OK’ I can make a PR with the color system copied to _variables.scss and the primary overridden to Sage’s traditional green.