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.

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.

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

Why exactly would caring about users lead you to omit something that makes things nicer for them?

Maybe the same reason why others choose to omit light modes, despite that being my preference.