* 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
Emit("did-start-loading");
}
void WebContents::DidStopLoading() {
  Emit("did-stop-loading");
}
void WebContents::DidGetResourceResponseStart(
    const content::ResourceRequestDetails& details) {
  Emit("did-get-response-details",
       details.socket_address.IsEmpty(),
       details.url,
       details.original_url,
       details.http_response_code,
       details.method,
       details.referrer,
       details.headers.get(),
       ResourceTypeToString(details.resource_type));
}
void WebContents::DidGetRedirectForResourceRequest(
    const content::ResourceRedirectDetails& details) {
  Emit("did-get-redirect-request",
       details.url,
       details.new_url,
       (details.resource_type == content::RESOURCE_TYPE_MAIN_FRAME),
       details.http_response_code,
       details.method,
       details.referrer,
       details.headers.get());
}
void WebContents::DidFinishNavigation(