This doesn't fix the issue of the bright flash but makes it shorter.
The `load` event is fired when the page and all resources are fully
loaded while the `DOMContentLoaded` event is fired when initial HTML
document has been completely loaded and parsed, without waiting for
stylesheets, images, and subframes to finish loading.
* 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>).
