Since the introduction in MacOS Mojave “Dark Mode” has made its way to the mainstream.
If a user chooses to use “Dark Mode” the overall experience of the OS adapts to provide a consistent user experience.
Many companies introduced a dark theme for their apps in 2018. Spark, Things 3, Sketch or Ulysses to name a few.
There is a website that has a comprehensive list of many more apps and websites. https://darkmodelist.com/.