Add dark mode support for article view
This MR adds dark mode support for the new article view.
I calculate this based on the difference between window and text color. There might be a better way, but this works between Breeze and Breeze Dark.
On dark mode, I found more natural to use the theme background color rather than an arbitrary dark. It feels more integrated. In light mode I chose to hardcode white as it felt better than the theme light gray background.
I also tweaked the scrollbar colors to use the same logic.