If dark mode is enabled some apps will automatically have it activated. You can use the Dark Mode for other apps, not just your Safari browser. So, if you have Mojave installed on your Mac, follow these steps to set the dark mode up. Previous versions had to use shortcuts and you had to make exceptions for every site you wanted to view in the dark mode.
Most Mac users had to wait for the macOS Mojave update to be able to activate the dark mode on their computers. Repeat the process for every website you want to read in dark mode.