Screenshot on README - it's Arc Darker
For Darker use:
Kvantum theme Arc Darker
Aurorae Arc Dark
Colorscheme Arc Light
Plasma theme Arc Dark
Darker - it's only dark toolbar and window decoration. Left sidebar impossible set dark, because window widget not support that - it's single area
There is a kvantum theme but no color scheme. Also, when I preview the darker kvantum theme the kvantum window turns white as in the light variant.