* 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 (guest_delegate_)
    guest_delegate_->SetSize(params);
}
bool WebContents::IsGuest() const {
  return type_ == WEB_VIEW;
}
bool WebContents::IsOffScreen() const {
#if defined(ENABLE_OSR)
  return type_ == OFF_SCREEN;
#else
  return false;
#endif
}
bool WebContents::IsOffScreenOrEmbedderOffscreen() const {
  return IsOffScreen() || (embedder_ && embedder_->IsOffScreen());
}
void WebContents::OnPaint(const gfx::Rect& dirty_rect, const SkBitmap& bitmap) {
  Emit("paint", dirty_rect, gfx::Image::CreateFrom1xBitmap(bitmap));
}
void WebContents::StartPainting() {
  if (!IsOffScreen())
    return;
#if defined(ENABLE_OSR)
  auto* osr_rwhv = static_cast<OffScreenRenderWidgetHostView*>(
      web_contents()->GetRenderWidgetHostView());
  if (osr_rwhv)
    osr_rwhv->SetPainting(true);
#endif
}
void WebContents::StopPainting() {