up:layer:opened

This event is emitted after a new overlay was placed into the DOM.

Customizing the overlay appearance

The event is emitted right before the opening animation starts. Because the overlay has not yet been painted by the browser, this is a good occasion to customize overlay elements:

up.on('up:layer:opened', function(event) {
  if (isChristmas()) {
    up.element.affix(event.layer.element, '.santa-hat', { text: 'Merry Christmas!' })
  }
})

See Customizing overlays for other methods to change the overlay appearance.