functions.php

<?php

/**
 * Register assets required by the theme.
 */
function pwcc_register_assets() {
    wp_register_style(
        'pwcc-styles',
        get_stylesheet_uri(),
        array(),
        '1.0'
    );
    wp_style_add_data( 'pwcc-styles', 'pwcc_h2_push', true );
}
add_action( 'wp', 'pwcc_register_assets' );

/**
 * Add HTTP Link headers required by server push.
 */
function pwcc_server_push() {
    global $wp_styles;

    foreach ( $wp_styles->registered as $style ) {
        if ( ! empty( $style->extra ) && $style->extra['pwcc_h2_push'] ) {
            header( 'Link: <' . esc_url_raw( $style->src ) . '>; rel=preload; as=style' );
            pwcc_style_cached( $style->handle );
        }
    }

    var_dump( $wp_styles );
}
add_action( 'wp','pwcc_server_push', 20 );

Hi,
Excuses where is "pwcc_style_cached" func? and why "var_dump"? :-D

tnx