Collect it while it’s available.
Nintendo will be releasing a new theme for Tetris 99. This time you’ll be able to unlock a Mario Golf: Super Rush theme with special art, music and Tetrimino designs once you’ve earned 100 event points.
The event will last from Friday 09-07-2021 up to 12-07-2021, though I don’t know the exact BST/CEST times yet.
If you’ve ever missed a previous theme you can exchange tickets to receive them.
Source: Nintendo Life