https://github.com/sanchothefat/wp-less/issues/43

plugin.php

<?php
/*
Plugin Name: Toolbar Theme Switcher - LESS CSS Integration
Plugin URI: https://github.com/sanchothefat/wp-less/issues/43
Description: Fix cache refresh when switching themes
Author: Leho Kraav
Author URI: http://leho.kraav.com
Version: 1.0
License: MIT
*/
function tts_less_init() {
    if ( ! class_exists( "Toolbar_Theme_Switcher" ) ) return;

    add_filter( "wp_less_cache_path", "tts_less_cache_path" );
    add_filter( "wp_less_cache_url", "tts_less_cache_path" );
}
add_action( "plugins_loaded", "tts_less_init" );

function tts_less_cache_path( $path ) {
    return sprintf( "%s-%s", $path, basename( get_stylesheet_directory() ) );
}
?>