* Add prefers-color-scheme support
This should fix <https://github.com/omarroth/invidious/issues/559>.
The cookie storage format has been changed from boolean
("true"/"false") to tri-state ("dark"/"light"/""), so that users
without a cookie set will get dark mode if they have enabled the dark
theme in their operating system. The code for handling the cookie
state, along with the user's operating system theme, has been factored
out into a new function `update_mode`, which is called both at window
load and at the "storage" event listener, because the "storage" event
listener is only trigerred when a change is made to the localStorage
from another tab/window (for more info - see
<https://stackoverflow.com/a/4679754>).
// If no preference indicated here and no preference indicated on the preferences page (backend), but the browser tells us that the operating system has a dark theme
set_mode(true);
}
// else do nothing, falling back to the mode defined by the `dark_mode` preference on the preferences page (backend)