Change Text Message Background 's Color by parameter

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
At the present the message backgorund on the chat component is set by the _reply_ attribute.

**Expected behavior:**
What I'm looking for is to change the background colour of a message if I marked it as favourite. I have added the property {favourite: true/false} on the message JSON input, but I'm not able to set the background colour basis on this parameter

**Steps to reproduce:**

I supose I have to change the property .text.background but I dont know how to enhance the class in order to have the property _favourite_ per message on the css

**Related code:**
` nb-chat-message-text {
align-items: flex-start;

.text {
@include nb-ltr(border-top-left-radius, 0);
@include nb-rtl(border-top-right-radius, 0);
background: nb-theme(chat-message-background);
color: nb-theme(chat-message-text-color);
}
}`

### Other information: