This is not official; just my comment:
It seems that the minimal change we need to make to have our custom themes be compatible with 2021.01 is to increase the width of one Theme Element:
Control Bar / Modules / Select / Background (large)
Increasing the width from 131px to 204px will allow the four new buttons to be visible in the UI. Otherwise, they are not shown except for a thin sliver on the right edge of the Select Module.
(It is not essential to customize the new butt