Inject alternate styles from a child theme into your theme's theme options stylesheet switcher.

inject-child-alt-styles.php

function woo_options_add ( $o ) {
	// If our options aren't an array, get out.
	if ( ! is_array( $o ) ) return $o;
	$theme_dir = get_option( 'template' );
	$style_dir = get_option( 'stylesheet' );
	// Loop through until we get to the woo_alt_stylesheet option.
	foreach ( $o as $k => $v ) {
		if ( !isset( $v['id'] ) || 'woo_alt_stylesheet' != $v['id'] ) continue;

		// Force a "select2" field type.
		$o[$k]['type'] = 'select2';
		// Transform our existing stylesheets to work with "select2".
		$styles = $v['options'];
		$o[$k]['options'] = array();
		foreach ( $styles as $i => $j ) {
			$o[$k]['options'][$j] = $j . ' (' . esc_attr( $theme_dir ) . ')';
		}

		if ( is_child_theme() ) {
			$styles = woo_custom_scan_for_styles( get_stylesheet_directory() );
			$o[$k]['options'] = array_merge( $o[$k]['options'], $styles );
		}
	}

	return $o;
} // End woo_options_add()

/**
 * Scan a specified directory for alt styles, and add the relative path to an array.
 * @param  string $dir The current child theme's directory.
 * @return array
 */
function woo_custom_scan_for_styles ( $dir ) {
	$styles = array();
	if ( is_dir( $dir ) ) {
	    if ($dir_open = opendir( $dir . '/styles/' ) ) {
	        while ( ($file = readdir( $dir_open ) ) !== false ) {
	            if( stristr( $file, '.css' ) !== false ) {
	                $styles['../../' . basename( $dir ) . '/styles/' . $file] = $file . ' (' . esc_attr( basename( $dir ) ) . ')'; // Add the full relative path, to avoid having to override another function.
	            }
	        }
	    }
	}
	return $styles;
} // End woo_custom_scan_for_styles()