I came here to post the same observation as OP. Used to be, you’d set default colors in your browser, and those would be used to render all web sites. No need to use a plug-in, override CSS per-site. If you wanted to, you could have the web site render into your own “brick wall” background.

Over time, browsers have ceded more and more control over layout and colors to web developers, and got busy burying/deprecating the browser-side defaults... to the point where user preference is an afterthought now, and almost universally ignored by sites. Now you have to use big hammers like disabling CSS and JavaScript in order to have any say as a user. Nobody likes when your native desktop application's UI ignores your system default colors and forces their own color scheme, yet this is acceptable on the web.