For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:doclai/openSUSE_Tumbleweed/home:doclai.repo
zypper refresh
zypper install arc-gtk-theme
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:doclai/15.5/home:doclai.repo
zypper refresh
zypper install arc-gtk-theme
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:doclai/15.4/home:doclai.repo
zypper refresh
zypper install arc-gtk-theme