Kindly follow the below mentioned steps to perform the above
Step 1: Kindly login to CometChat Admin Panel
Step 2: Browse through the left tab, and look for settings
Step 3: Look for editor tab and select custom css from the drop down available on right top.
Step 4: Add below code in Custom CSS
Step 5: Make sure Custom CSS option must be enabled on that window
Step 6: Lastly, clear the Admin panel Cache and your Browser cache using the clear cache tab and browser settings.