We want to fully customize the Chat Widget rather than use the out of the box widget. Is it possible to customize the CSS for the Chat Widget?
No, it is not possible to edit the CSS of the Chat Widget. If you want to fully customize the widget, use our Zendesk Chat Web SDK. The Web SDK allows you to take control of the look and feel of the Chat Widget.
For more information about the Zendesk Chat Web SDK, see the article: Web SDK documentation.