Introduces a new config element and environment variable
(WHOOGLE_CONFIG_THEME) for setting the theme of the app. Rather than
just having either light or dark, this allows a user to have their
instance use their current system light/dark preference to determine the
theme to use.
As a result, the dark mode setting (and WHOOGLE_CONFIG_DARK) have been
deprecated, but will still work as expected until a system theme has
been chosen.
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