Add a custom message to the customizer's main panel above the controls.

wpse313731-custom-message.css

#customize-theme-controls .control-panel-themes.wpse313731-has-custom-message {
	margin-bottom: 15px;
}
#customize-theme-controls .control-panel-themes.wpse313731-has-custom-message > .accordion-section-title {
	margin-bottom: 0;
}
#customize-theme-controls .control-panel-themes.wpse313731-has-custom-message .wpse313731-custom-message {
	background: white;
	border-bottom: 1px solid #ddd;
	padding: 10px 12px 10px 12px;
}

wpse313731-custom-message.js

( function( $ ) {

	'use strict';

	wp.customize.bind( 'ready', function () {
		wp.customize.panel( 'themes', function( panel ) {
			panel.deferred.embedded.done( function() {
				var customMessage;
				panel.headContainer.addClass( 'wpse313731-has-custom-message' );
				customMessage = $( wp.template( 'wpse313731-custom-message' )() );
				panel.headContainer.append( customMessage );
			} );
		} );
	} );
} )( jQuery );

wpse313731-custom-message.php

<?php
/**
 * Plugin Name: WPSE313731 Custom Message
 * Description: Add a custom message to the customizer's main panel above the controls.
 * Plugin URI: https://gist.github.com/timelsass/d7c0ca7d50a7b92681e9936adbba423a
 * Author: Tim Elsass
 * Author URI: tim.ph
 */
 
/**
 * Enqueue CSS and JS for customizer pane.
 */
function wpse313731_enqueue_scripts() {
	$handle = 'wpse313731-custom-message';
	wp_enqueue_script( $handle, plugins_url( "$handle.js", __FILE__ ), array( 'customize-controls' ) );
	wp_enqueue_style( $handle, plugins_url( "$handle.css", __FILE__ ) );
}

add_action( 'customize_controls_enqueue_scripts', 'wpse313731_enqueue_scripts' );

/**
 * Print custom message template.
 */
function wpse313731_print_templates() {
	?>
	<script type="text/html" id="tmpl-wpse313731-custom-message">
		<p class="wpse313731-custom-message"><?php esc_html_e( 'This is a custom message being added to the WordPress Customizer.' ) ?></p>
	</script>
	<?php
}

add_action( 'customize_controls_print_footer_scripts', 'wpse313731_print_templates' );