I didn't change anything. I installed the theme and when I tried to apply it this error popped up. I even tried with a fresh copy of Aseprite without any extension or script installed.
This issue is most likely related to the font chosen for the theme. It’s one of the default ones but it seems to only work on Windows, I switched to a Unicode version of the Arial font. You can try the build here.
If this solves the problem, I’ll release it as an official update for the theme. ✌️