cosmic-ext-bg-theme from X11:COSMIC:Backports project

对于 15.6，请以根用户 root 运行下面命令：

zypper addrepo https://download.opensuse.org/repositories/X11:COSMIC:Backports/15.6/X11:COSMIC:Backports.repo
zypper refresh
zypper install cosmic-ext-bg-theme