Get theme automatically from system setting

Uses CSS `prefers-color-scheme` to get the user's preferred colour
scheme. Also bundles up some theme logic into its own class.

src/settings/Settings.js

@@ -275,6 +275,11 @@ export const SETTINGS = {
         supportedLevels: LEVELS_ACCOUNT_SETTINGS,
         default: [],
     },
+    "use_system_theme": {
+        supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS,
+        default: true,
+        displayName: _td("Match system theme"),
+    },
     "webRtcAllowPeerToPeer": {
         supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS_WITH_CONFIG,
         displayName: _td('Allow Peer-to-Peer for 1:1 calls'),