Though it's not specified in the episode itself, the theme for the graybles is the Five Stages of Grief, as outlined by Elizabeth Kubler-Ross.
1. Denial: Cinnamon Bun complains about the "denial of light."
2. Anger: the Lemongrabs fight over Lemonsweets (the only one in which the name of the stage is not spoken).
3. Bargaining: Ice King makes a bargain with the Penguins so he can watch the movie he wants.
4. Depression: the Fox comments on the depression in his bed which is the exact shape of his body.
5. Acceptance: Finn pretends to sign the form to "accept" the package from Jake.