Toolbar button to toggle between two set themes

[skhzhang]

From a review left by a user on AMO: https://addons.mozilla.org/en-CA/firefox/addon/automatic-dark/reviews/1799763/

"would it be possible in a future update to include a toolbar button that could be clicked to toggle back and forth between the two set themes? On very dark winter days, even though sunset is still several hours away, my office gets so dark that I find that I need to activate the dark theme early. A one-click solution for that scenario would be highly appreciated!"