According the Core.php EE library, the config needs to be:
$config['theme_folder_url'] = $base_url . "/assets/third_themes/";
But latest versions of EE add the new third_party themes var:
$config['url_third_themes'] = $base_url . "/assets/third_themes/";
But also keep in mind that EE will automatically append "third_party" to this. So for example the final URL will be "example.com/assets/third_themes/third_party/redactee/".
Changing folder structure for 100% compatibility
If you want to make things compatible for addons that do and dont support the new variable you can structure your assets folder to the following:
And then set both the config vars to the following:
$config['theme_folder_url'] = $base_url . "/assets/themes/";
$config['url_third_themes'] = $base_url . "/assets/themes/third_party/";
This should ensure all addons work with the new third_party themes location, regardless of supporting the url_third_themes var.
This solution will involve moving the EE CP theme folders into the assets folder as well, but thats the price you pay to ensure 100% compatibility without editing 3rd party addons.