* 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
web_contents()->StopFinding(action);
}
void WebContents::ShowDefinitionForSelection() {
#if defined(OS_MACOSX)
  const auto view = web_contents()->GetRenderWidgetHostView();
  if (view)
    view->ShowDefinitionForSelection();
#endif
}
void WebContents::CopyImageAt(int x, int y) {
  const auto host = web_contents()->GetMainFrame();
  if (host)
    host->CopyImageAt(x, y);
}
void WebContents::Focus() {
  web_contents()->Focus();
}
#if !defined(OS_MACOSX)
bool WebContents::IsFocused() const {
  auto view = web_contents()->GetRenderWidgetHostView();
  if (!view) return false;
  if (GetType() != BACKGROUND_PAGE) {
    auto window = web_contents()->GetNativeView()->GetToplevelWindow();
    if (window && !window->IsVisible())
      return false;
  }
  return view->HasFocus();
}
#endif
void WebContents::TabTraverse(bool reverse) {