It’s a normal browser setting left over from the early days of the Internet, when most pages did little or no theming (or did it badly). My primary browser is Pale Moon, which exposes the setting as a dropdown under Preferences > Content > General > Colors. Firefox also exposed the setting on its settings page the last time I checked, although it may be a checkbox there. It’s also a boolean in about:config, browser.display.use_document_colors.
Chrome-family browsers may not have such a setting, unfortunately. The only one I keep handy is Vivaldi, which doesn’t have it but does have a “Force a dark theme on all websites” checkbox (under Settings > Appearance > Website Appearance).


You can install any desktop environment on any distro. For some of the more obscure DEs on some distros, a bit of hoop-jumping may be required, but it’s still possible. And some distros ship without any DE at all.
Technically, you can have a functional desktop without a DE at all, just a bare window manager (in X) or compositor (wayland), but that’s getting into advanced topics.