Hi, Try to add this code in the CSS Adjustment tab :
.wa_agent_header, .wa_agent_header_tab {
background-color:black!important}
.offline-contact-form form .btn-submit {
background:black!important;}
https://www.dropbox.com/s%2Fm9v9d2j6zsp1u9p%2FModules%2520mod_whatsapp_agent%2520-%2520j39%2520-%2520Administration%25202020-06-13%252006-10-31.png?raw=1