wp-content/themes/customtheme/style.css

/*
    Theme Name: Custom Theme
    Theme URI: https://wordpress.org/
    Author: procoder
    Author URI: https://wordpress.org/
    Description: Custom Theme Tutorial
    Version: 1.0
    License: GNU General Public License v2 or later
    License URI: http://www.gnu.org/licenses/gpl-2.0.html
    Tags: black
    Text Domain: customtheme
*/