Support OS preference for light or dark mode

[willpower232]

Unfortunately I don't have direct access to this OS feature so this is more of a to-do. If not directly set, the default result should be dark mode.

• Make light_mode boolean nullable as a third state to allow the app to use auto detection

• Refer to https://www.kirupa.com/tricks/dealing_with_dark_mode.htm and add javascript to update light mode with OS setting change

• Create API endpoint to set light mode from javascript and save as a session value then update the view composer to use the session value if the user value is null

• Update toggle in profile to use indeterminate state and set the value to null in existing api endpoint then add in session value setting and figure how to determine what the actual light_mode state should be

[willpower232]

More desktop browser support is coming:

https://bugzilla.mozilla.org/show_bug.cgi?id=1494034
https://bugs.chromium.org/p/chromium/issues/detail?id=889087

Ref and more ideas: https://justmarkup.com/log/2019/02/adapting-to-user-preferences/

[willpower232]

Seems like its supported now, can use something like matchMedia('(prefers-color-scheme: dark)').matches natively.