Understanding the WordPress template hierarchy - Use the template hierarchy to change the presentation of a specific page or a bunch of them or all of them. It's your choice! https://www.damiencarbery.com/2019/10/understanding-the-wordpress-template-hierarchy/

index.php

<?php
wp_head();

if ( have_posts() ) {
	while ( have_posts() ) {
		the_post();
		the_title();
		the_content();
	}
}
wp_footer();

style.css

/*
Theme Name: Minimal Experiment
Theme URI: https://www.damiencarbery.com/2019/10/understanding-the-wordpress-template-hierarchy/
Author: Damien Carbery
Author URI: https://www.damiencarbery.com
Description: Basic theme with minimal files.
Version: 1.0
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
*/