In order for you to How to use Custom CSS, Custom JS & integration from Admin Panel.
Login to CometChat Admin panel using credentials.
From left menu Click on "Settings->Editor"
Before starting any changes copy "restore.php" file URL. so if anything went wrong at the time of editing above file and CometChat stop working just run "restore URL".
Make a change as per your requirement and click on "save"
From left menu "Clear Cache"
Clear browser Cache.
Check if your changes are reflected on site.