Be able to customize background colors for User and Chatbot in conversational AI

**Is your feature request related to a problem? Please describe.**
I couldn't find a way to set up the background color of the conversational AI solution found [here](https://labelstud.io/templates/slot_filling.html).

**Describe the solution you'd like**
A clear way to configure the background color for User and Chatbot message so that the label colors can be chosen in way so that they're visible and clear.

Maybe there's is a way to do it, but I couldn't find it.