Ones was, i think, qtwebbrowser with the possibility to switch to dark-mode for the different web-pages, today is no one web-browser based on Qt that allow or have a dark-mode.
i like the konqueror because block almost every annoying thing on the web, but… dark web-surfing is not there.
Also some other “addons” can be useful on konqueror & Co.
Have you intention to do something about this topic?