In Story/Arcade Mode there are stages that are not found in the stage select in Multiplayer/Training, this mod allows you to select them outside their Mode

• VelvetBeautifier [gamebanana.com] (A mod loader/tool)
  
• Hidden Stages [gamebanana.com] (the mod)

• Download Velvet Beautifier and extract it somewhere on your computer (you might have to install .NET 8.0 runtime for Velvet Beautifier)
  
  GUI version
  If you have downloaded the GUI version follow these steps:
  
  1. Open up Velvet Beautifier (VelvetBeautifier.GUI.<your-os>), you should be greeted with this screen after a while:
     
     
  2. Go to File -> Install Mod -> From text
     
  3. Copy-Paste the mod link and click Install, you should see a mod listed there if you refresh, if not try again
     
  4. Check the mod by clicking on the checkbox and click Apply
     
  5. Profit
     
  6. ???
     
  7. Congrats, you just installed a mod, now you can launch the game and enjoy the seasonal lobbies
  CLI version
  If you have downloaded the CLI version follow these steps:
  
  1. Launch a Terminal where the CLI is located and let it run by itself for initial setup
     
  2. After that execute the following command to install one of the mods:
     
     VelvetBeautifier.CLI --install <link-to-the-mod>
  3. Then enable the mod by executing this command:
     
     VelvetBeautifier.CLI --enable ntf.tmh.hiddenstages
  4. To apply the mods execute the final command:
     
     VelvetBeautifier.CLI --apply
  5. Profit
     
  6. ???
     
  7. Congrats, you just installed the mod, you can now go playing on these stages

One cool fact about this mod is that it's client-side! This means that nobody has to install this mod online to play on these stages too. The only downside is that they might not hear any music during the stage and they cannot select the stage, you can only do that