[`mgt-chat`] Support for customized theming

[sebastienlevert]

[ghost]

Hello sebastienlevert, thank you for opening an issue with us!

I have automatically added a "needs triage" label to help get things started. Our team will analyze and investigate the issue, and escalate it to the relevant team if possible. Other community members may also look into the issue and provide feedback 🙌

[gavinbarron]

After some digging it looks like we can pull any fluent theming that may be added above the chat components via the context.

There is sample code that does something similar to what we need to do here: https://codesandbox.io/p/sandbox/testing-for-v8-and-v9-themes-oclepm?file=%2Fsrc%2FApp.tsx%3A34%2C13