* 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()) {
#if defined(ENABLE_OSR)
    auto* osr_rwhv = static_cast<OffScreenRenderWidgetHostView*>(
      web_contents()->GetRenderWidgetHostView());
    if (osr_rwhv)
      osr_rwhv->Invalidate();
#endif
  } else {
    const auto window = owner_window();
    if (window)
      window->Invalidate();
  }
}
gfx::Size WebContents::GetSizeForNewRenderView(
    content::WebContents* wc) const {
  if (IsOffScreen() && wc == web_contents()) {
    auto relay = NativeWindowRelay::FromWebContents(web_contents());
    if (relay) {
      return relay->window->GetSize();
    }
  }
  return gfx::Size();
}
void WebContents::SetZoomLevel(double level) {