Alright, the loot boxes are now integrated and covers all the items of the game. Here are some examples:
You have 40% chance to get only diamonds. And the 60% left are distributed between colors, tie colors, health upgrades, dash upgrades, clothes, swords, guns, masks, and super attacks. Of course you have more chances to get a new color than a new sword (20% vs 5%).
The player needs to get 3 stars on a level to unlock a loot box.