Do you mean the “Night Light” in the “Displays” settings? It only adjusts the color temperature of your screen to filter out blue light. It is not intended to change the theme or the color scheme.
Assuming that your Pure OS is older than Crimson, the toggle to change the color scheme is not yet in the Settings application, but you should still be able to change the theme and the color scheme of the applications, including browsers. There is a method for Byzantium in another post.
If you mean the contents on the web pages as well, many implement support for changing the colors in CSS when the host color scheme changes. However, if you want to rewrite CSS automatically and enforce a dark appearance on all websites, even if they have not implemented one, you would need a browser with WebExtensions support and the Dark Reader extension.
If you install browsers through Flatpak, you would need a newer version of xdg-desktop-portal-gtk than packaged in PureOS Byzantium.
The web engine behind GNOME Web (Epiphany) is WebKit, not Blink. To be more precise, it is WebKitGTK, the GTK port of WebKit. It is also a dependency for many other GNOME applications.