Midnight - KDE Plasma.
For better looking please use this theme with Kvantum engine.
(Kvantum is an SVG-based theme engine for Qt4/Qt5, KDE and LXQt, with an emphasis on elegance, usability and practicality.)
You can find the official installation instruction here (Github).
Midnight : Full theme urls (opendesktop.org) :
- Plasma Look-and-Feel
- Plasma Color Scheme
- Plasma Theme
- Aurorae Theme
- Kvantum Theme
- SDDM Login Theme
- Midnight on Github