Additional backgrounds can be unlocked for the title screen "sandbox" mode by entering the words below:

2 - "cat"
3 - "car"
4 - "bee"
5 - "tree"
6 - "woman"
7 - "coffin"
8 - "vibes"
9 - "coin"
10 - "chair"
11 - "zombie"
12 - "court"
13 - "rain" and select "rain(water)"
14 - "It"
15 - "pc"