Converting HTML Template to WordPress Theme

As said, you should enqueue your style sheets like this in your functions.php in your theme:

function adds_to_the_head() { // Our own unique function called adds_to_the_head

wp_register_style( 'custom-style-css', get_stylesheet_directory_uri() . '/css/styles.css','','', 'screen' ); // Register style.css

wp_enqueue_style( 'custom-style-css' ); // Enqueue our stylesheet

}
add_action( 'wp_enqueue_scripts', 'adds_to_the_head' );