It's a fun game. But unfortunately, the window needs to be active for the game to run; if you switch to another window, the game pauses.
This means there's no way to earn rewards offline.
Thank you for the feedback!
I updated the game, so it can now continue running in the background as long as it remains the active tab in the browser window. Unfortunately, browsers may still throttle or pause the Unity Web Player when another tab becomes active, so full offline progress is not supported yet.