[Sync] Give custom themes priority over default themes

chrome/browser/themes/theme_syncable_service.cc

 // Copyright (c) 2012 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.
 
 #include "chrome/browser/themes/theme_syncable_service.h"
 
 #include "base/strings/stringprintf.h"
 #include "chrome/browser/extensions/extension_service.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/themes/theme_service.h"
@@ skipping 68 matching lines @@
     // save the unsyncable theme to sync data.
     return merge_result;
   }
 
   // Find the last SyncData that has theme data and set the current theme from
   // it.
   for (syncer::SyncDataList::const_reverse_iterator sync_data =
       initial_sync_data.rbegin(); sync_data != initial_sync_data.rend();
       ++sync_data) {
     if (sync_data->GetSpecifics().has_theme()) {
-      MaybeSetTheme(current_specifics, *sync_data);
-      return merge_result;
+      if (!current_specifics.use_custom_theme() ||
+          sync_data->GetSpecifics().theme().use_custom_theme()) {
+        MaybeSetTheme(current_specifics, *sync_data);
+        return merge_result;
+      }
     }
   }
 
   // No theme specifics are found. Create one according to current theme.
   merge_result.set_error(ProcessNewTheme(
       syncer::SyncChange::ACTION_ADD, current_specifics));
   return merge_result;
 }
 
 void ThemeSyncableService::StopSyncing(syncer::ModelType type) {
@@ skipping 232 matching lines @@
       syncer::SyncChange(FROM_HERE, change_type,
                          syncer::SyncData::CreateLocalData(
                              kCurrentThemeClientTag, kCurrentThemeNodeTitle,
                              entity_specifics)));
 
   DVLOG(1) << "Update theme specifics from current theme: "
       << changes.back().ToString();
 
   return sync_processor_->ProcessSyncChanges(FROM_HERE, changes);
 }