`theme` backfill in user profile fields doesn't occur if the profile already exists but doesn't contain `darkMode`

[clpetersonucf]

In get_profile_fields(): https://github.com/ucfopen/Materia/blob/master/app/core/models.py#L1528

If the "theme" is empty and "darkMode" is not present, theme needs to be included. This doesn't appear to break anything, but we should probably populate it with the default value.