* 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
atom::api::WebContents::Type* out) {
    using Type = atom::api::WebContents::Type;
    std::string type;
    if (!ConvertFromV8(isolate, val, &type))
      return false;
    if (type == "backgroundPage") {
      *out = Type::BACKGROUND_PAGE;
    } else if (type == "browserView") {
      *out = Type::BROWSER_VIEW;
    } else if (type == "webview") {
      *out = Type::WEB_VIEW;
#if defined(ENABLE_OSR)
    } else if (type == "offscreen") {
      *out = Type::OFF_SCREEN;
#endif
    } else {
      return false;
    }
    return true;
  }
};
}  
namespace atom {
namespace api {
namespace {
content::ServiceWorkerContext* GetServiceWorkerContext(