[v5] [Storybook] Fix some stories appear broken on devices which prefer dark mode

[slax57]

The following stories appeared broken when rendered on a device which prefers dark mode:

• http://localhost:9010/?path=/story/ra-ui-materialui-auth-logout--user-authenticated : icon button appears missing
• http://localhost:9010/?path=/story/ra-ui-materialui-button-savebutton--basic : button is not visible at all
• http://localhost:9010/?path=/story/ra-ui-materialui-button-togglethemebutton--legacy : toggle theme button is rendered twice => I guess it's because we no longer support BC in v5, so I simply removed the story

This is because the color scheme applied is intended for dark backgrounds, but it is currently white.
Adding a MUI Paper fixes the issue.

[fzaninotto]

The problem is that, in dark mode, MUI components expect to be rendered on a dark background. Instead of forcing the theme, we should render these stories on a Paper to force a dark background in dark mode.

[slax57]

The problem is that, in dark mode, MUI components expect to be rendered on a dark background. Instead of forcing the theme, we should render these stories on a Paper to force a dark background in dark mode.

Fixed