To Add External Atylesheet:
Generally speaking, you should use the wp_enqueue_style() function to add external stylesheet to your theme.
function wpse259600_add_style() {
wp_enqueue_style( 'theme_critical', get_template_directory_uri() . '/css/critical.css', array(), '1.0.0' );
}
add_action( 'wp_enqueue_scripts', 'wpse259600_add_style' );
To Add Internal Stylesheet:
However, if (for whatever reason) you have to add internal stylesheet (i.e. printing the entire CSS file within the HTML <head> tag using the <style> tag), then you may use inclue_once like below:
add_action( 'wp_head', 'wpse259600_internal_css_print' );
function wpse259600_internal_css_print() {
echo '<style type="text/css">';
// this will also allow you to use PHP to create dynamic css rules
include_once get_template_directory() . '/css/critical.css';
echo '</style>';
}
This will not generate warning in theme check. Although, I wouldn’t recommend this. If you want to add custom CSS this way, you better save it to the database with customizer theme options and then add it with wp_add_inline_style function along with your theme’s main CSS file.