If the chat for the site according to the given criteria?

Question

API

Good afternoon, colleagues!
I am looking for a chat for site visitors with the following criteria:

• Availability of a PC client
  
• Availability of a mobile application for operators
  
• Presence of a simple API for event handling
  
• Possibility to customize the dialog window interface
  

I have already found many similar services, but nowhere did I see the possibility of customizing the dialog box. Is there such a thing? Has anyone seen it on GitHub?