The Waxed theme has been updated in line with the Boost changes for Moodle 3.5 and Bootstrap 4 final.
https://moodle.org/plugins/theme_waxed
https://github.com/roelmann/moodle-theme_waxed
This includes updated Bootstrap4Stable Bootswatch Presets which may be used in any Boost3.5 theme.
As before, Waxed is kept as a deliberately simple child of Boost with only a few small changes and some additional settings.
It is (in my opinion ) an ideal theme to use as a base to begin your own theme developments based on Boost.