[Improvement][Main Container][Text] Replace variants that will be removed in the next major release in material-ui
According to news info regarding material-ui, a "flat" variant will be removed in the next update, most probably leading to text based issues.
Please consider updating currently existing variants to address this update.
```Failed prop type: The `flat` variant will be removed in the next major release. `text` is equivalent and should be used instead.```
```Material-UI: you are using the deprecated typography variants that will be removed in the next major release.```