I don't know that a website needs a dark mode. Does anyone really not visit a website because it doesn't match their desktop color theme? That seems absurdly picky to me.
I guess my real issue (which I forgot to mention previously) is: why is this something that the website needs to implement? Shouldn't a personal preference such as dark mode, like reader mode, be something browsers implement instead of the individual websites?
On the other hand, people accustomed to using bright mode turn down their monitor brightness and have no issues. As a rule, the white background should not be much brighter on a monitor than a piece of paper next to it.
This often results in brightness settings between 15-20%.
This weird contrived strawman scenario is an insult to people with disabilities.