Theme Boost Union is an enhanced child theme of Boost which is intended, on the one hand, to make Boost simply more configurable and, on the other hand, to provide helpful additional features for the daily Moodle operation of admins, teachers and students.
This PR is a no-brainer. As soon as the tests have passed, it can be integrated.
However, we won't know if it really solves the problem which is described in #674 before the next release on moodle.org