doesn't work for me either. it says that it cannot open a file at С:\Users\Username\AppData\Roaming\Aseprite\extensions\theme-preferences\./Template.lua, even though it exists in said folder. it didn't prompt me with permission, but i had some extensions installed before, so maybe i already granted all the permissions, idk. not sure how this part works.
i use 1.3 beta10, windows10 64bit