In order to install the theme and add it to your decorations do the following:

git clone https://github.com/jomada/SierraBreeze
cd SierraBreeze
mkdir build
cd build
cmake .. -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_BUILD_TYPE=Release -DKDE_INSTALL_LIBDIR=lib -DBUILD_TESTING=OFF -DKDE_INSTALL_USE_QT_SYS_PATHS=ON
sudo make install

To avoid logging out, issue

kwin_x11 --replace &

That is it! Your new decoration theme should appear in Settings → Application Style → Window Decorations.

Acknowledgments:

• The authors of Breeze window decorations Martin Gräßlin and Hugo Pereira Da Costa
• Andrey Orst, the author of Breezemite Aurorae window decoration https://github.com/andreyorst/Breezemite
• Chris Holland for his blog about patching Breeze decorations https://zren.github.io/projects/2017/07/08/patching-breeze-window-decorations.html