* Electron version: 1.7.5

### Expected behavior

When a theme color is no longer present the webContents did-change-theme-color event should return `null`.

### Actual behavior

The event returns `"#000000"`.

### How to reproduce

1. Log the returned color of the webContents did-change-theme-color event.
2. Go to https://electron.atom.io which has a theme color.
3. Go to https://atom.io which does not have a theme color.




atom/browser/api/atom_api_web_contents.cc
zoom_controller_->SetZoomLevel(level);
}
double WebContents::GetZoomLevel() {
  return zoom_controller_->GetZoomLevel();
}
void WebContents::SetZoomFactor(double factor) {
  auto level = content::ZoomFactorToZoomLevel(factor);
  SetZoomLevel(level);
}
double WebContents::GetZoomFactor() {
  auto level = GetZoomLevel();
  return content::ZoomLevelToZoomFactor(level);
}
void WebContents::OnSetTemporaryZoomLevel(double level,
                                          IPC::Message* reply_msg) {
  zoom_controller_->SetTemporaryZoomLevel(level);
  double new_level = zoom_controller_->GetZoomLevel();
  AtomViewHostMsg_SetTemporaryZoomLevel::WriteReplyParams(reply_msg, new_level);
  Send(reply_msg);
}
void WebContents::OnGetZoomLevel(IPC::Message* reply_msg) {