WordPress Including internal css file


Example

In this case style.css is located in root of the theme's folder

function themeSlug_enqueue_scripts() {
    wp_enqueue_style( 'themeSlug-style', get_template_directory_uri() .'/style.css', '1.0.0');
}
add_action('wp_enqueue_scripts', 'themeSlug_enqueue_scripts');