I want to apply css targeting class in material UI sx prop

 <Dialog
  fullScreen
  open={addMedicineDlg}
  onClose={handleAddMedicineDlgClose}
  aria-labelledby={"add-medicine-dialog"}
  TransitionComponent={Transition}
  disableEnforceFocus
  sx={{
    "& .ck-rounded-corners .ck.ck-balloon-panel, .ck.ck-balloon-panel.ck-rounded-corners":
      { zIndex: 9999 },
  }}
>

But it is not working. Can anyone point out where am I doing mistake