Schedule background in line with dark mode schedule

## Problem

I typically use a light wallpaper during the day and manually switch to a dark wallpaper when dark mode kicks in.

## Proposal

Allow the user to choose what wallpaper to set during day and night. This may require some design changes in how the settings is shown to the user since the wallpaper and colour mode selection are on different views.

## Prior Art

I _think_ macOS changes to a darker background when dark mode kicks in: