Is there a way to add a custom background please ?

Hello there better Message Team, please my client said they would love to have the chat displayed like on a glass background, which believe adding a column ,make the background look like glass by adding a class image as the column background image and then add the better Message shortcode would be the fastest way of achieving their request, but then it overrided , it didn’t work, can please help me out with any code or idea please?