Browser background too white

You may have already tried this, but have you tried turning down the screen brightness (there is a brightness adjustment in the top row of keyboard keys).

I haven't tried the following but I suspect that while it may work it also may be a bit cumbersome. See the following from Use Dark Mode on your Mac - Apple Support

• Safari automatically shows a website in Dark Mode if the website has been designed to support it. If the website doesn't support Dark Mode, you can use Safari Reader to read articles in Dark Mode.