Add help text to the end of the Custom CSS section in the Customizer.

add-to-custom-css-help-text.php

<?php
add_action( 'customize_register', function( $wp_customize ) {
	$custom_css_section = $wp_customize->get_section( 'custom_css' );
	if ( $custom_css_section ) {

		// @todo The section-description-buttons should really not be part of the description help text.
		$custom_css_section->description = preg_replace(
			'/(?=<p class="section-description-buttons)|$/',
			'<p>' . __( 'Here are some additional instructions for the theme, for example certain class names or IDs to target.' ) . '</p>',
			$custom_css_section->description,
			1
		);
	}
}, 11 );