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function wpse_enqueue_page_template_styles() {
    if ( is_page_template( 'mytemplate.php' ) ) {
        wp_enqueue_style( 'page-template', get_template_directory_uri() . '/css/page-template.css' );
    }
}
add_action( 'wp_enqueue_scripts', 'wpse_enqueue_page_template_styles' );