Breeze Dark variant, with opaque backgrounds and without shadows.
There is also a AUR (Arch Linux) package: https://aur.archlinux.org/packages/breeze-dark-flat
This theme links to up-to-date, system resources. However, System Settings replaces the links by copies. It is advised either to remove and reinstall the theme with System Settings with each major update of Plasma; or extract manually the archive so that `breeze-dark-flat` folder is in `~/.local/share/plasma/desktoptheme/`.
See light variant: https://store.kde.org/p/1215691/