for gtk apps make sure you have breezy-gtk (for default plasma) or breeze-gtk (plasma 5.6 or higher).
Then set this to be used for gtk apps.
Some apps - like chrome - would need to have a specific theme installed.
Yast installer runs as root - so you’d need to specify a dark theme for root as well - but even then, I’m not sure that yast follows any user set theme - it may be a fixed theme.