Edit oembedded contents

Change excerpt length

function wpa_oembed_excerpt_length() {
  return 20;
}
add_filter( 'rest_oembed_output_excerpt_length', 'wpa_oembed_excerpt_length', 9 );

Change oembedded content style

function wpa_oembed_output() { ?>
  <style>
    .wp-embed {
      font-family: serif !important;
    }
  </style>
<?php }
add_action( 'rest_oembed_output', 'wpa_oembed_output', 11 );