Add a user agent stylesheet for extension options pages

extensions/renderer/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # GYP version: extensions/extensions.gyp:extensions_renderer
 source_set("renderer") {
   sources = [
     "activity_log_converter_strategy.cc",
     "activity_log_converter_strategy.h",
     "api_activity_logger.cc",
@@ skipping 62 matching lines @@
     "render_view_observer_natives.h",
     "request_sender.cc",
     "request_sender.h",
     "resource_bundle_source_map.cc",
     "resource_bundle_source_map.h",
     "resources/app_runtime_custom_bindings.js",
     "resources/binding.js",
     "resources/context_menus_custom_bindings.js",
     "resources/entry_id_manager.js",
     "resources/event.js",
+    "resources/extension.css",
     "resources/extension_custom_bindings.js",
+    "resources/extension_fonts.css",
     "resources/greasemonkey_api.js",
     "resources/i18n_custom_bindings.js",
     "resources/image_util.js",
     "resources/json_schema.js",
     "resources/last_error.js",
     "resources/messaging.js",
     "resources/messaging_utils.js",
     "resources/permissions_custom_bindings.js",
     "resources/platform_app.css",
     "resources/platform_app.js",
@@ skipping 68 matching lines @@
       "send_request_natives.cc",
     ]
   }
 
   if (is_win) {
     cflags = [
       "/wd4267", # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     ]
   }
 }