Improve DefaultTheme.js @mui/system

[Akshat111111]

As an EmbeddedChat developer

I need to: Implementing the thing using mui/system create theme function.

So That: It will be more compatible with material UI.

Acceptance Criteria

• [ ] TODO 1
• [ ] TODO 2
• [ ] TODO 3

[abhinavkrin]

Hey @Akshat111111 , we don't use material UI for EmbeddedChat. So this change is not required for the project.