* 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
return static_cast<AtomBrowserContext*>(web_contents()->GetBrowserContext());
}
void WebContents::OnRendererMessage(const base::string16& channel,
                                    const base::ListValue& args) {
  
  Emit(base::UTF16ToUTF8(channel), args);
}
void WebContents::OnRendererMessageSync(const base::string16& channel,
                                        const base::ListValue& args,
                                        IPC::Message* message) {
  
  EmitWithSender(base::UTF16ToUTF8(channel), web_contents(), message, args);
}
mate::Handle<WebContents> WebContents::CreateFrom(
    v8::Isolate* isolate, content::WebContents* web_contents) {
  
  auto existing = TrackableObject::FromWrappedClass(isolate, web_contents);
  if (existing)
    return mate::CreateHandle(isolate, static_cast<WebContents*>(existing));
  
  return mate::CreateHandle(isolate, new WebContents(isolate, web_contents,
        REMOTE));
}
mate::Handle<WebContents> WebContents::CreateFrom(