In SP!NG, stars are earned by collecting jewels in each level. Collecting all the jewels that are available in the level will earn you 3 stars.

Once you have unlocked all available themes, then for every 40 stars earned, you can unlock a new character from the currently equipped theme. For example, if you wanted to earn the Ghost character in the PARK L!FE theme, then you should activate the PARK L!FE theme as you play in order to receive a new random character from PARK L!FE. 

We are currently actively exploring new and fun ways to earn stars, which should get you earning more cool characters and themes in future updates!