no idea if we're talking about browsing by pc or smartphone ... .
I use an addon Dark Background and Light Text for my browser and it works like a charme at my pc's and can easily tweaked to my liking.
But i agree webdesign without barriers for different needs is rare to find.
Using different templates/stylesheets shouldn't be to hard to do, but nowadays it seems to be more important to have a new cookie or tracking script .... every other day.
Have a nice weekend