ui: calendar popover doesn't apply typography

[Forfold]

Expected behavior: MUI typography styles are applied within calendar popover components.

[dctalbot]

I did some investigating here and there's not a simple solution unfortunately. We can't modify the default popup, but we can render our own component which might be the way to go anyway.

If we just want to patch the styles, we can create an .rbc-overlay class, but I'm not sure how we could do that and still pull in styles from our theme provider 🤔