It can be done poorly as well, like in Facebook / Google apps on Apple devices. They feel very unintegrated into the rest of the OS.

It's not necessarily _that_ easy to do, especially depending on how your CSS was implemented (eg; do you use variables for all your colors, and do they have a sane, consistent, reversible brightness scale?)

However, I certainly agree with your "you do this because you _care_" sentiment.

Source: implemented dark mode for an application where the "sane css colors using variables" property was true.

What if I care specficially not to add a dark mode?

There's a large button. It loads before the text does. When the text loads, you'll see in big letters, right next to the button, Dark Mode, italicization and all. If you click on the button, suddenly you get a very blue-purple dark mode experience.