For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce/openSUSE_Tumbleweed/X11:xfce.repo
zypper refresh
zypper install adwaita-xfce-icon-theme
For openSUSE Slowroll run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce/openSUSE_Slowroll/X11:xfce.repo
zypper refresh
zypper install adwaita-xfce-icon-theme
For openSUSE Factory run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce/openSUSE_Factory/X11:xfce.repo
zypper refresh
zypper install adwaita-xfce-icon-theme
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce/15.6/X11:xfce.repo
zypper refresh
zypper install adwaita-xfce-icon-theme
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce/15.5/X11:xfce.repo
zypper refresh
zypper install adwaita-xfce-icon-theme
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce/15.4/X11:xfce.repo
zypper refresh
zypper install adwaita-xfce-icon-theme