Make chat colours custom

[zeucxb]

Nowadays, chat use primary and accent colours as base, but it's not always true. Sometimes you want to use a different color to your chat page, or just to your chat response background.

Would be awesome have a ChatStyle or something to define the custom style

Something like: AppBarSettings -> here that must be a model.

[davidfranquet]

Opened a pull request - #6

[zeucxb]

@DavidFD005 thank you, I add some comments to code review.