cpoy the comment in the index.css in the default theme to your index.css and edit it.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>
<?php bloginfo('name') ?>
</title>
</head>
<body>
<header>
<?php echo (bloginfo('name')) ?>
</header>
<main>
<?php if (have_posts()) : ?>
<?php while (have_posts()) : ?>
<?php the_post(); ?>
<h1>
<?php the_title(); ?>
</h1>
<div>
<?php the_content(); ?>
</div>
<?php endwhile ?>
<?php else : ?>
<?php endif; ?>
</main>
<footer>
footer
</footer>
</body>
</html>header.php
<!DOCTYPE <?php language_attributes(); ?>>
<html lang="en">
<head>
<meta charset="<?php bloginfo('charset') ?>">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>
<?php bloginfo('name') ?>
</title>
<?php wp_head(); ?>
</head>
<body>
<header>
<a href="<?php echo (home_url()) ?>">
<?php echo (bloginfo('name')) ?>
</a>
</header>index.php
<?php get_header() ?>
<main>
<?php if (have_posts()) : ?>
<?php while (have_posts()) : ?>
<?php the_post(); ?>
<h1>
<?php the_title(); ?>
</h1>
<div>
<?php the_content(); ?>
</div>
<?php endwhile; ?>
<?php else : ?>
<?php endif; ?>
</main>
<?php get_footer() ?>footer.php
<footer>
<span><?php bloginfo('name') ?></span> <?php echo (date('Y')) ?>
</footer>
</body>
<?php wp_footer(); ?>
</html>add this code to the functions.php
<?php
function enqueue_theme_resources()
{
wp_enqueue_style('style', get_stylesheet_uri());
}
add_action('wp_enqueue_scripts', 'enqueue_theme_resources');