On fedora (the version I used is currently 28) the path doesn't exist, I had to change the path with the correct one that is /boot/grub2/themes.
It would probably be better to use an alternative path like /usr/share/grub/themes
In this case, if the copy of the files was not successful with the previous command (as it happened in my case, not finding the path), specifying an incorrect path will not find any theme when the grub will be updated.
cp -a StylishDark /boot/grub/themes
On fedora (the version I used is currently 28) the path doesn't exist, I had to change the path with the correct one that is /boot/grub2/themes. It would probably be better to use an alternative path like /usr/share/grub/themes
echo "GRUB_THEME=\"/boot/grub/themes/StylishDark/theme.txt\"" >> /etc/default/grub
In this case, if the copy of the files was not successful with the previous command (as it happened in my case, not finding the path), specifying an incorrect path will not find any theme when the grub will be updated.
elif has_command grub2-mkconfig; then
grub2-mkconfig -o /boot/grub/grub.cfg
In this case, it isn't considered an alternative path for uefi systems which, in my case, is /boot/efi/EFI/fedora/grub.cfg
To solve the problem, I had to change the various paths manually before running the script. Install.txt