At the moment, in the settings home page only Breeze dark and Breeze light can be chosen. When you select one of them and apply, the (ptentially chosen manually by user) theme-specific settings (decoration, mouse pointer etc) as well as the theme are nuked and Breeze dark/light is applied instead. It would be nice if it just allowed to apply "Dark" or "Light" user-specified themes (if a theme allows to mark itself as light/dark and allows to specify a light/dark variant (variant name I guess)). In case the current theme (before apply) is Breeze, keep user changes for theme-specific settings so that they're not nuked (for instance, window decoration etc).
Technically that UI isn't restricted to Breeze. Set DefaultLightLookAndFeel / DefaultDarkLookAndFeel in kdeglobals under [KDE] >(if a theme allows to mark itself as light/dark and allows to specify a light/dark variant Unfortunately this isn't something that exists.