Example using wp_body_open hook to add a top banner

add_top_banner.php

<?php // Ignore this first line when copying to your child theme's functions.php file.

add_action('wp_body_open', function() { ?>
	<style>
		.top-banner {
			background-color: firebrick;
			color: white;
			display: grid;
			font-size: 2rem;
			height: 10vmin;
			place-items: center;
			width: 100vw;
		}
	</style>
	<div class="top-banner">This is my cool Top Banner!</div>
<?php } );

/**
 * You can add the PHP code snippet to your child theme's functions.php file 
 * or with third-party plugins such as My Custom Functions and Code Snippets.
 * /