add_action( 'wp_head', 'gt_co_early_hide', -999 );
function gt_co_early_hide() {
    if ( ! is_checkout() ) { return; }
    echo '<style>body.woocommerce-checkout,html body.woocommerce-checkout{background:#000!important;}body.woocommerce-checkout #header-outer,body.woocommerce-checkout #header-space,body.woocommerce-checkout #slide-out-widget-area,body.woocommerce-checkout #slide-out-widget-area-bg,body.woocommerce-checkout .ocm-effect-wrap,body.woocommerce-checkout .nectar-quick-view-box-backdrop,body.woocommerce-checkout .slide_out_area_close,body.woocommerce-checkout #header-secondary-outer,body.woocommerce-checkout .nectar-sticky-header-wrap{display:none!important;background:#000!important;opacity:0!important;visibility:hidden!important;pointer-events:none!important;}</style>';
}