custom.php

    function wrap_youtube_autoembed_in_flexbox($html, $url) {
        if (stripos($url, "youtube") !== false) {
            $html = '<div class="flex-video widescreen">' . $html . '</div>';
        }

        return $html;
    }

    add_filter("oembed_result", "wrap_youtube_autoembed_in_flexbox", 10, 2);