Ionic 4: Toast messages not accepting any css style

I’m currently using the toastController to create a toast this way:

      message: '<span style="color: red">Be careful, </span>' + 'you do not have any access here.',
      animated: true,
      cssClass: 'custom-white-font',
      color: 'success',
      position: 'top'
    }).then(toast => toast.present());

However, the style of the span inside the message isn’t used by the toast at all. It doesn’t even appear in the HTML page, except the span tag which stands alone with the text inside.
Did I miss something to get my css used? Or there is just not any way to add some css only in a part of the message?

Thank you.