Did you install it through the extensions tab inside Preferences?
I have no experience with Linux, so I have to be honest I can't give you much expertise here. :)
Viewing post in Aseprite Dark Mort HD Theme comments
Yes, I read the instructions and did exactly that. Everything just flashes black for 0.5 of a second and changes back to normal. I set the zip file to read and write just Incase but that didn’t help. I don’t mind the default theme anyway so it’s no worry. Some people who use Aseprite might be starting to use Linux soon though since Win 7 lost support, but it’s probably a very small number of people who will happen to use Linux, use Aseprite and want a black theme.
Thanks for replying though!