There was a logical error in the compile_sass management command:
instead of falling back to settings.COMPREHENSIVE_THEME_DIRS when
--theme-dirs was *None or missing*, we only fell back to it when
--theme-dirs was *missing*.
This caused theme compilation to be skipped when COMREHENSIVE_THEME_DIRS
*is not set* in the environment, even though
settings.COMPREHENSIVE_THEME_DIRS *is set* in Django settings, which
is currently the case for edx.org.