Automatically switch based on time of day

[isantop]

Oftentimes Dark mode is not used as a hard definitive preference but rather as a dynamic adjustment to ambient lighting conditions. Usually at night there are lower levels of ambient light, requiring darker content on screens.

It would be good to allow changing the mode dynamically based on whether Night Light is enabled.

[mmstick]

We would need:

• A patch for gnome-settings-daemon's color plugin to apply changes on sunrise / sunset.
• An update to the schema for the color plugin for the ambient setting.
• A small patch here to add a widget to the UI for setting that setting.