I love this. Any website where I am expected to spend any decent amount of time needs to have a dark mode. Very surprised so few websites support this.
Supporting two different CSS templates can add a lot of work, sometimes it's simpler just to officially support one.
I understand that and I'm definitely not expecting it from every website but if you have a website that basically depends on me spending a lot of time reading or doing something on that web page then you should try to not make it a horrible experience for me.