Since I'm working on a review, I decided to make a thheme based on the green colors of my YouTube channel. Far from perfect, because, well, I'm not an artist, but good enough for my needs to show how it's modular.
[REPLACEME] name=Tsundoku Zone 0=d0e0d3 1=ffffff 2=74aa7c 3=498c57 4=74aa7c 5=a5c3ac
Notably, on Mac, I've found it easiest to just locate the game, Open Package Contents, and find the pallettes.ini file to edit.