I'm most familiar with the U.S., where it's not a requirement. Europe is the only place where I've heard there might be a requirement to honor a device's color scheme, I think it's very unlikely it exists anywhere else.
If a site honors the dark/light setting, I don't know what accessibility need would be served by being in the opposite mode for that one site. Having a toggle to meet preferences, yes, need, I don't know.
If a site honors the dark/light setting, I don't know what accessibility need would be served by being in the opposite mode for that one site. Having a toggle to meet preferences, yes, need, I don't know.