[Sync] Give custom themes priority over default themes

chrome/browser/themes/theme_syncable_service.cc

Index: chrome/browser/themes/theme_syncable_service.cc
diff --git a/chrome/browser/themes/theme_syncable_service.cc b/chrome/browser/themes/theme_syncable_service.cc
index 5b8a2d8344e4d49c5b63a36e05bb9b8fa3f383bd..09fd2a1da64ad2649aaa52e1ab1044060f71ad66 100644
--- a/chrome/browser/themes/theme_syncable_service.cc
+++ b/chrome/browser/themes/theme_syncable_service.cc
@@ -86,8 +86,11 @@ syncer::SyncMergeResult ThemeSyncableService::MergeDataAndStartSyncing(
       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;
+      }
     }
   }