Ensure the settings page accurately represents theme choices

Fixes https://github.com/vector-im/riot-web/issues/11518

This also fixes a bug where the the theme logic can incorrectly get stuck in the light theme - the ThemeController was overriding *all* values, not just supposed themes. Null values aren't overridden so that the various theme logic bits don't assume the user has chosen the light theme explicitly.

src/theme.js

@@ -80,6 +80,8 @@ export class ThemeWatcher {
     }
 
     getEffectiveTheme() {
+        // Dev note: Much of this logic is replicated in the GeneralUserSettingsTab
+
         // If the user has specifically enabled the system matching option (excluding default),
         // then use that over anything else. We pick the lowest possible level for the setting
         // to ensure the ordering otherwise works.