Somehow the behavior has changed in how Discourse handles CSS in theme components. EDIT I did not understand what I was doing. I’m leaving this topic (in Dev instead of Bug) for future reference.
Very minimal example:
On Discourse 3.4.2 and tests-passed as of this morning (but this was also happening yesterday on 3.4.1)
Main theme #1: red border around navigation container
Theme component #2: blue border around navigation container
Theme component #2 is assigned to theme #1
Expected
Blue border around navigation container
Reality
Red border around navigation container