For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:glaffite/openSUSE_Tumbleweed/home:glaffite.repo
zypper refresh
zypper install grub-customizer
For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:glaffite/16.0/home:glaffite.repo
zypper refresh
zypper install grub-customizer
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:glaffite/15.6/home:glaffite.repo
zypper refresh
zypper install grub-customizer