We built several custom themes based on Lightning. At some point, I believe with the Salesforce Sprint '17 update, all of those themes switched to looking like they were based on Modern. Can't figure out how to fix. Some of the themes looked right in our Sandbox which was running Brooklyn, while Production wasn't running Brooklyn. When we upgraded Production to Brooklyn, the themes didn't get fixed. I do have Update Out of Date Themes showing, but it just runs and runs, I've tried leaving it for 15 minutes and it never completes. I can have the theme looking right in Sandbox, and create a new theme in Production with the same files, and it looks to be based on Modern instead of Lightning.