Enqueue preferred and alternative style sheets.

functions.php

<?php

add_action( 'wp_enqueue_scripts', 'child_add_alternate_style_sheet' );
/**
 * Enqueue preferred and alternate style sheet.
 *
 * WordPress already adds a persistent style sheet (style.css), so this
 * function gives examples of adding a preferred and alternative style sheets.
 *
 * Persistent = browser MUST apply it.
 * Preferred = browser SHOULD apply it, unless user has selected a different alternative.
 * Alternative = browser should allow users to select from these mutually exclusive style sheets.
 *
 * @author Gary Jones
 * @link   https://garyjones.io/enqueued-style-sheet-extras
 * @see    https://www.w3.org/TR/html4/present/styles.html#h-14.3.1
 */
function child_add_alternate_style_sheet() {
	global $wp_styles;
	
	// Add a prefered style sheet, for the normal contrast (colours, font sizes etc)
	wp_enqueue_style( 'child-default', get_stylesheet_directory_uri() . '/style-default.css', [], '1.0' );
	$wp_styles->add_data( 'child-default', 'title', __( 'Default', 'child-theme-textdomain' ) );
	
	// Add an alternative style sheet, high contrast.
	wp_enqueue_style( 'child-highcontrast', get_stylesheet_directory_uri() . '/style-highcontrast.css', [], '1.0' );
	$wp_styles->add_data( 'child-highcontrast', 'title', __( 'High Contrast', 'child-theme-textdomain' ) );
	$wp_styles->add_data( 'child-highcontrast', 'alt', true );
	
}