How to edit chat window header background

Even though there is no option to edit the chat window header by default, you are able to edit it using Custom CSS. To do so, navigate to Configuration > Chat > Chat buttons > Edit your chat button. Then in the "Chat window" tab scroll down to the "Custom CSS" section and add the following CSS code:

.InfoPanel {
    background-color: black !important;
    border-top-left-radius: 8px;
    border-top-right-radius: 8px;

Do not forget to APPLY the CSS code and SAVE the chat window.