This is the Spectrum Minimal Classic theme locked with the Breeze Dark color scheme so it stays dark regardless of the color scheme you are using. Use this if you want your panel and widgets to be dark, but to have different colored windows.
This plasma theme is used in Spectrum Dark
Have you tried turning it off and on again? Sometimes plasma assets will not update and some may still be themed according to something you previously had set. This can usually be fixed by restarting.
To restart just plasmashell, open KRunner with "Alt + F2" to execute the following 2 lines: