I have managed to regress it with mozregression and came up with this result:
"2019-11-12T11:27:28: INFO : platform_version: 68.0a1
2019-11-12T11:28:12: DEBUG : Starting merge handling...
2019-11-12T11:28:12: DEBUG : Using url: https://hg.mozilla.org/integration/mozilla-inbound/json-pushes?changeset=824a86797c8ecbb1a6395ac9fd76ad6abe724766&full=1
2019-11-12T11:28:13: DEBUG : Found commit message:
Bug 1525762: Part 1j - Work around XPI omni.jar packaging issues. r=bustage CLOSED TREE
We have some convoluted packaging logic to automatically package any directory
with a manifest.json file as an XPI. Unfortunately, that logic also applies to
extensions that are supposed to be part of omni.ja.
It would be nice to have a cleaner way to filter out any resources which we've
already flagged to be part of omni.ja, but this patch takes the simpler
approach of just including anything that's in a modules/ directory, which is
whitelisted for omni.ja elsewhere.
2019-11-12T11:28:13: DEBUG : Did not find a branch, checking all integration branches
2019-11-12T11:28:13: INFO : The bisection is done.
2019-11-12T11:28:13: INFO : Stopped"
I have to mention that I had to skip the last 2 builds that mozregression threw at me because it would not have the Windows dark theme applied at all, so I could not determine whether they're good or a bad builds. In any case, I believe the result is correct.