I feel like dark mode is a sign that people care. I think it's a user feature that is easy to do, but not done often. Mostly done by people who care. It brings so much polish to the experience.
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.
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.