lila/bin/mongodb/pref-bg-migrate.js

db.pref.find({ dark: { $exists: 1 } }, { dark: 1, transp: 1 }).forEach(p => {
  const bg = p.dark ? (p.transp ? 400 : 200) : 100;
  db.pref.update({ _id: p._id }, { $set: { bg: NumberInt(bg) }, $unset: { dark: 1, transp: 1 } });
});