You can tweak the sprites however the goal of the jam is to use the game assets you're provided with - that's the challenge. If you'd rather not use the game assets I'd recommend joining a different jam.
From what I understand, if you can tweak the sprites within Unity that is allowed, like overlaying a different color or something. But seriously, Kenney has so many assets that everyone can pick a different set and there would be very little number of games with overlapping assets...
