Enqueue styles in Theme

wds_enqueue_styles_in_theme.php

<?php
function my_theme_enqueue() {
    $version = filemtime( get_stylesheet_directory() . '/css/styles.css' ) ;
    wp_register_style( 
        'my_theme_main_css', 
        get_template_directory_uri() . '/css/styles.css', 
        array(), 
        $version
    );
    wp_enqueue_style( 'my_theme_main_css' );
}

add_action( 'wp_enqueue_scripts', 'my_theme_enqueue' );