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/zh-CN.json

@@ -68,7 +68,11 @@
     "Show related videos: ": "显示相关视频？",
     "Show annotations by default: ": "默认显示视频注释？",
     "Visual preferences": "视觉选项",
+    "Player style: ": "",
     "Dark mode: ": "暗色模式：",
+    "Theme: ": "",
+    "dark": "",
+    "light": "",
     "Thin mode: ": "窄页模式：",
     "Subscription preferences": "订阅设置",
     "Show annotations by default for subscribed channels: ": "在订阅频道的视频默认显示注释？",