Luckily quite a few commercial websites do include a dark mode, such as YouTube and Facebook. This dark mode will force a black background. Super Dark Mode is an extension for Firefox which will attempt to switch on a website dark mode, if the option exists. This can look super-bright on your eyes if the rest of the user-interface is black. There are problems and not even application will work as smoothly, particularly your email client and web browser, although the UI will switch to the correct mode, will often show a bright white background as it's fixed by the HTML code provided by either the incoming or web page. Indeed, some people keep dark mode on permanently. These modes make the general user-interface easier on the eye. Both Windows 10 and Mac OS now support a night or dark mode.
