During the development period and tweaking and theming of Plasma lateron, we used to have qt5ct or QT5 Configuration Tool to set our Plasma desktop settings like fonts, icons, …
There is one setting in particular that is important if you want to keep using QT5CT and you want to have a Breeze dark.
That is what this page and video is about and only that.
Take a look at the images above.
This one setting will either break the settings page of desktop effect or make it on your Breeze dark.
System Settings – Desktop Effects