These are the Plasma themes for the Simply Grey color schemes.
There is two Plasma themes. One for the two lighter color schemes and one for the darkest variant
The color you see in the screenshots is the the Simply Dark Grey color scheme.

The color schemes can be found here:
https://www.pling.com/p/1306837

The Newaita icons match this theme perfectly and they can be found here:
https://store.kde.org/p/1243493