plasma-theme-oxygen from KDE:Applications:18.04 project
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Add repository and install manually
For openSUSE Leap 15.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/KDE:Applications:18.04/openSUSE_Leap_15.0/KDE:Applications:18.04.repo zypper refresh zypper install plasma-theme-oxygen