How can I add border and background color to threads?

Please add the below custom CSS code in your theme’s customize setting. For this, please go to Dashboard > Appearance > Themes > Customize > Additional CSS. Please note, this will affect only the frontend, to change the CSS in the backend you need to use the “Simple Custom CSS and JS” plugin. 

.wpsc-thread.reply.agent{
background-color: antiquewhite;
padding: 5px;
border: 1px;
border: solid;
}

.wpsc-thread.reply.customer{
background-color: #d7fadb;
padding: 5px;
border: 1px;
border: solid;
}