Loading styles and scripts into your WordPress theme

<?php 

add_action('wp_enqueue_scripts', function() {
	<!-- here we link our files -->
}
?>

<?php 

add_action('wp_enqueue_scripts', function() {
	wp_enqueue_style(
		'style-name', <!-- file name (you're free to name it whatever you want) -->
		<!-- file path -->
                get_stylesheet_directory_uri() . '/style.css', // Path
        )
}
?>

function my_theme_assets() {
    // CSS
    wp_enqueue_style(
        'main-style', // Handle
        get_stylesheet_directory_uri() . '/style.css', // Path
        [], // Dependencies
        filemtime(get_stylesheet_directory() . '/style.css') // Version (cache bust)
    );

    // JS
    wp_enqueue_script(
        'main-script', // Handle
        get_template_directory_uri() . '/script.js', // Path
        ['jquery'], // Dependencies
        filemtime(get_template_directory() . '/script.js'), // Version
        true // Load in footer
    );
}
add_action('wp_enqueue_scripts', 'my_theme_assets');

Styles and scripts for dashboard customization

function my_admin_assets() {
    wp_enqueue_style('admin-style', get_template_directory_uri() . '/admin.css');
    wp_enqueue_script('admin-script', get_template_directory_uri() . '/admin.js', [], false, true);
}
add_action('admin_enqueue_scripts', 'my_admin_assets');