Are theme cruises by definition done with cruise resellers?

Or do some of the cruise lines do them directly?

If the cruise line sponsors the theme cruise it is open to everyone and it is covered in your cruise rate...if a promoter or private group sponsors a theme private event on board it is open only to the private group and the promoter will charge a surcharge..you will pay your cruise fare and surcharge to the promoter, who in turn will pay the cruiseline the agreed to rate..other non group cruisers will not be allowed admission, to the private events, but the private group can attend both the private activities and the regular ship activities,,,if the promoter fails to provide advertised events, the cruiseline is not liable..if it is a full charter theme cruise, the promoter can charge what the market will bear and all shipboard events and facilities are in the promoters control including ports of call...the week that the vessel is chartered, the cruiseline will not allow the general public to purchase a cabin and the ship will not show up on the lines website

From time to time you will read a story on these boards where a cruiser will complain that they reserved and paid for a specific week on a specific ship and are advised by yhe cruiseline that a private group has taken over (full charter) that vessel for that week and the cruise line will offer you an alternative vessel (usually with an upgrade or onboard credit) as an incentive or a full refund and credit for a future cruise...this is rare and usually means that ship was very slow selling and the cruiseline had enough excess capacity fleet wise to absord everyone...