Both `theme` and `lastDarkTheme` sync, so a theme chosen on one machine
-- and the toggle's way back to it -- are the same everywhere. That is
already true, by the rule that DEVICE_KEYS is a list of exceptions and
anything else syncs by default, but nothing said so.
The existing test cannot say it: it derives what should sync from
DEVICE_KEYS, so moving one of these into that list would move the
expectation with it and still pass. These name the two keys outright.
jcoffey-dev
95e56303f7