When the Breeze Dark look-and-feel/desktop-theme is applied, colour schemes for widgets are ignored - panel backgrounds and highlight colours are locked to the default Breeze Dark colours of blue on dark grey. The standard Breeze (light) widgets honour colour schemes as expected.
That's by design. There's "Breeze" which follows the color schemes, "Breeze Light" which is always light and "Breeze Dark" which is always dark. Use "Breeze" and set a dark color scheme.
Created attachment 113697 [details] Breeze Dark color scheme applied to Breeze theme Okay, though that is quite confusing. I guess my problem then is that the "Breeze" forces light panel backgrounds, regardless of what the color scheme defines - see the attached screenshot of the Breeze Dark color scheme applied to "Breeze" and of the "Breeze Dark" panels.
Created attachment 113698 [details] Breeze Dark color scheme applied to Breeze Dark theme