Since the release of Safari Technology Preview 68 in October, Apple has been building support for the use of ‌Dark Mode‌ CSS, which can be enabled via its Develop -> Experimental Features menu.

Version 55 of Opera's web browser for Android will come with a night mode that not only dims the browser, but will turn white web pages dark even if they're not designed that way.

With the Safari included in the 10.14.4 update the same thing will happen for websites with a dark theme, with the right CSS set up. Have a look at how it works here: ...