Add prefers-color-scheme support (#601)

* 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>).

locales/eu.json

@@ -68,7 +68,11 @@
     "Show related videos: ": "",
     "Show annotations by default: ": "",
     "Visual preferences": "",
+    "Player style: ": "",
     "Dark mode: ": "",
+    "Theme: ": "",
+    "dark": "",
+    "light": "",
     "Thin mode: ": "",
     "Subscription preferences": "",
     "Show annotations by default for subscribed channels: ": "",