Add an option to have a "Sepia mode"

[juliemoynat]

In the app, there is an "Absolute black mode" option for dark theme. But the light mode is in absolute white by default with no option to change that colour. The absolute white background tires my eyes so I'd like to have a "sepia mode" like in the Firefox reader view where the background is in beige #F4ECD8 colour.

Solution: Add an option to have a "Sepia mode" that makes the background beige #F4ECD8 instead of white.