Add docs and flag for custom theme support

docs/labs.md

@@ -98,3 +98,12 @@ person is online.
 
 This adds padlocks to room list tiles and room header for invite only rooms.
 This feature flag (unlike most) is enabled by default.
+
+## Custom themes (`feature_custom_themes`)
+
+Custom themes are possible through Riot's [theme support](./theming.md), though
+normally these themes need to be defined in the config for Riot. This labs flag
+adds an ability for end users to add themes themselves by using a URL to the JSON
+theme definition.
+
+For some sample themes, check out [aaronraimist/riot-web-themes](https://github.com/aaronraimist/riot-web-themes).

riot.im/develop/config.json

@@ -24,7 +24,8 @@
         "feature_invite_only_padlocks": "enable",
         "feature_event_indexing": "disable",
         "feature_bridge_state": "labs",
-        "feature_presence_in_room_list": "labs"
+        "feature_presence_in_room_list": "labs",
+        "feature_custom_themes": "labs"
     },
     "welcomeUserId": "@riot-bot:matrix.org",
     "piwik": {