embed-content.phpはこんな感じにした。

<?php

/**
 * Contains the post embed content template part
 *
 * When a post is embedded in an iframe, this file is used to create the content template part
 * output if the active theme does not include an embed-content.php template.
 *
 * @package WordPress
 * @subpackage Theme_Compat
 * @since 4.5.0
 */
?>
<style>
.wp-embed {
	width: 95%;
	height: 95%;
	max-width: 750px;
	padding: 10px;
	border: none;
}
.wp-embed-featured-image img {
	object-fit: cover;
	width: 80px;
	height: 80px;
}
p.wp-embed-heading {
	font-size: 14px;
	-webkit-line-clamp: 2;
	-webkit-box-orient: vertical;
	overflow: hidden;
}
.wp-embed-excerpt {
	font-size: 11px;
	display: -webkit-box;
	-webkit-line-clamp: 3;
	-webkit-box-orient: vertical;
	overflow: hidden;
}
.wp-embed-site-icon {
	width: 12px;
	height: 12px;
}
.wp-embed-site-title a {
	padding-left: 15px;
	font-size: 10px;
}
.wp-embed-footer {
    display: table;
    width: 100%;
	margin :0px;
}
</style>
<div <?php post_class('wp-embed'); ?>>
  <?php

  /**
   * Filters the thumbnail image ID for use in the embed template.
   *
   * @since 4.9.0
   *
   * @param int $thumbnail_id Attachment ID.
   */
    /**
     * Filters the thumbnail image size for use in the embed template.
     *
     * @since 4.4.0
     * @since 4.5.0 Added `$thumbnail_id` parameter.
     *
     * @param string $image_size   Thumbnail image size.
     * @param int    $thumbnail_id Attachment ID.
     */

	/**
     * Filters the thumbnail shape for use in the embed template.
     *
     * Rectangular images are shown above the title while square images
     * are shown next to the content.
     *
     * @since 4.4.0
     * @since 4.5.0 Added `$thumbnail_id` parameter.
     *
     * @param string $shape        Thumbnail image shape. Either 'rectangular' or 'square'.
     * @param int    $thumbnail_id Attachment ID.
     */
  ?>

    <?php /*
      <div class="wp-embed-featured-image rectangular">
      */ ?>
    <div class="wp-embed-featured-image square">
      <a href="<?php the_permalink(); ?>" target="_top">
			<img src="<?php echo catch_that_image(); ?>" alt="<?php the_title(); ?>" />
      </a>
    </div>

  <p class="wp-embed-heading">
    <a href="<?php the_permalink(); ?>" target="_top">
      <?php the_title(); ?>
    </a>
  </p>

  <?php if ($thumbnail_id && 'square' === $shape) : ?>
    <div class="wp-embed-featured-image square">
      <a href="<?php the_permalink(); ?>" target="_top">
			<img src="<?php echo catch_that_image(); ?>" alt="<?php the_title(); ?>" />
      </a>
    </div>
  <?php endif; ?>

  <div class="wp-embed-excerpt"><?php the_excerpt_embed(); ?></div>

		<?php
		/**
		 * Prints additional content after the embed excerpt.
		 *
		 * @since 4.4.0
		 */
		do_action( 'embed_content' );
		?>

		<div class="wp-embed-footer">
			<?php the_embed_site_title(); ?>

			<div class="wp-embed-meta">
				<?php
				/**
				 * Prints additional meta content in the embed template.
				 *
				 * @since 4.4.0
				 */
				do_action( 'embed_content_meta' );
				?>
			</div>
		</div>
</div>