Improvement to dark mode selection

[andybee]

As I understand the current dark mode implementation looks at the current browser/OS setting first and sets that as a default value, before letting the user customise via the top right toggle.

I have my OS set to flip from light to dark based on sunset/sunrise, so if I visit the site for the first time during the day I'm on the light theme and have to toggle it at sunset to dark, then back to light again the next day etc. (curse you, short daylight of winter!)

Could we add a third option for "OS/Browser default", or not persist a default setting and always check the browser's opinion on page load?