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).