The plugin does not sanitise and escape some of its settings, which could allow high privilege users such as admin to perform Stored Cross-Site Scripting attacks even when the unfiltered_html capability is disallowed (for example in multisite setup)
1. In the plugin's settings, add the payload `"><script>alert(1)</script>` to the "Custom CSS Classes" field.
2. Save and when the page reloads, see XSS.