* 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
content::WebContents* source,
    const content::OpenURLParams& params) {
  if (params.disposition != WindowOpenDisposition::CURRENT_TAB) {
    if (type_ == BROWSER_WINDOW || type_ == OFF_SCREEN)
      Emit("-new-window", params.url, "", params.disposition);
    else
      Emit("new-window", params.url, "", params.disposition);
    return nullptr;
  }
  
  if (Emit("will-navigate", params.url))
    return nullptr;
  
  
  if (IsDestroyed())
    return nullptr;
  return CommonWebContentsDelegate::OpenURLFromTab(source, params);
}
void WebContents::BeforeUnloadFired(content::WebContents* tab,
                                    bool proceed,
                                    bool* proceed_to_fire_unload) {
  if (type_ == BROWSER_WINDOW || type_ == OFF_SCREEN)
    *proceed_to_fire_unload = proceed;
  else
    *proceed_to_fire_unload = true;
}
void WebContents::MoveContents(content::WebContents* source,