Expand reader background to cover non-safe area as well

[chrigi]

Is your feature request related to a problem? Please describe.
As discussed in #263 the reader background (when not using true-black mode) does not cover the screen fully, making it look a bit awkward.

Describe the solution you'd like
The non-safe areas should have the same color as the reader background.

Describe alternatives you've considered
This does not necessarily mean that the active reader view has to go above the safe-space. It would be fine if the pages cut off at the safe space and only the color spans the whole screen. At the moment only MacBooks come to mind where the notch might eat into the content in fullscreen if the reader would span the whole screen.

Additional context
It's only a nitpick, nothing important.