I think allowing it to automatically deduce one would be nice. One issue is
how well could a custom low contrast OS theme be detected. I think at least
the option would need to be exposed. Perhaps under content or maybe a new
accessibility tab would make sense.
The setting would be global. A site could allow the user to override it
within the site itself if they so choose. I don't think the browser needs
to concern itself with the theme, just the user's preference.
A good point on CSS. I am not very knowledgeable, but that does seem like a
logical approach.
I think though some major sites would implement it. At least enough to
drive some interest. For smaller sites I can see it being a burden,
especially when doing updates since the changes needv to be made across all
themes.
I can see the desire to avoid adding yet another feature especially if it
isn't widely adopted. I am not aware of any study that would shed any light
on how many people prefer light on dark for example. I know there is at
least some movement towards dark UI themes recently (Windows 8 metro ui,
Gnome 3, Android), so I think there might be something to it.
- Sepand