You can play offline by going to the Community page and clicking Steam on the upper left, then choose "Go Offline". Steam will restart. After it does go to JNPG (I use my icon in the System Tray) and click it. When the game opens choose to play Solo. You'll have access to all the courses but you are not logged into Steam to be able to play any tournaments or Multiplayer rounds.
1. Steam Community
2. Steam Link upper left
3. Choose Go Offline
4. Steam will restart
5. Launch JNPG
6 Choose Solo play
7. You'll see all the courses in the Carousel
Steam on top left side of Community page.
Here's how I have customized Steam in my System Tray.