As to how dark modes should work, I think that the best tools look at the applied/rendered background colors and figure out if they exist within a range of "dark" or "light" hues. Then, use that determination to decide if it should invert or not. It's certainly not an exact science. However, more and more sites are providing dark modes and using detection of OS settings to determine what to do. This feature could perhaps detect if the site is doing that as well and then react accordingly.