Lucien_Lachance I think this is because SolusDark is incomplete - there is no GTK SolusDark, so it falls back to Breeze (Dark).
An issue with KDE is that, if certain elements (e.g. fonts) in the KDE theme are changed, the equivalent elements in the GTK theme have to be changed by hand - they do not automatically change to reflect the KDE settings.
There are also KDE elements of SolusDark missing - e.g. no SDDM login screen or splash screen.
However, despite these flaws, SolusDark is an excellent start and the basics are there.