flat-plat-theme from home:PerryWerneck:gnome project
Select Your Operating System
Add repository and install manually
For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:PerryWerneck:gnome/openSUSE_Tumbleweed/home:PerryWerneck:gnome.repo zypper refresh zypper install flat-plat-theme