if( ! function_exists('marstheme_video_responsive' ) ){
    function marstheme_video_responsive( $html ) {
        return '<div class="video-container">' . $html . '</div>';
    }

    add_filter( 'embed_oembed_html', 'marstheme_video_responsive', 10, 3 );
    add_filter( 'video_embed_html', 'marstheme_video_responsive' ); // Jetpack
}