zIndex Theme override/make additions

I would like the ability to either add new properties to the zIndex theme object or modify the currently available keys:

    mobileStepper: 1000,
    appBar: 1100,
    drawer: 1200,
    modal: 1300,
    snackbar: 1400,
    tooltip: 1500

for example, I would like to add popover: 1000 but cannot find a way to add it to the theme. The last I could find online regarding this was for older versions of the library and are now obsolete on our version 4.4.2, https://stackoverflow.com/questions/35071435/set-component-z-index-on-custom-theme-in-material-ui.

If I have missed this somewhere in the documentation, please point me in the right direction. I was only able to find the bit about changing the values (not adding or modifying the keys), https://material-ui.com/customization/z-index/

Thanks very much and appreciate your work!

[oliviertassinari]

Right, so people can then do:

.MuiPopover-root {
  z-index: 1000 !important;
}

or with the theme

const theme = createMuiTheme({
  overrides: {
    MuiPopover: {
      root: {
        zIndex: '1000 !important',
      }
    }
  }
});

[combstraight]

Is it possible to set zIndex in overrides within createMuiTheme using '!important'? Currently trying to override MuiDialog’s zIndex using createMuiTheme but failing to do so since the property does not accept strings.