* 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
if (!IsOffScreen())
    return;
#if defined(ENABLE_OSR)
  auto* osr_rwhv = static_cast<OffScreenRenderWidgetHostView*>(
      web_contents()->GetRenderWidgetHostView());
  if (osr_rwhv)
    osr_rwhv->SetPainting(false);
#endif
}
bool WebContents::IsPainting() const {
  if (!IsOffScreen())
    return false;
#if defined(ENABLE_OSR)
  const auto* osr_rwhv = static_cast<OffScreenRenderWidgetHostView*>(
      web_contents()->GetRenderWidgetHostView());
  return osr_rwhv && osr_rwhv->IsPainting();
#else
  return false;
#endif
}
void WebContents::SetFrameRate(int frame_rate) {