Introduces the same functionality as PR #4294

How to test:
edit template xml [isis.xml] file adding
<field name="compile" type="compile" label="TPL_ISIS_COMPILE_LABEL" /> under a fieldset
create a folder fields and a file in that fodder named compile.php
and paste this code:

<?php
/**
 * @package     Joomla.Administrator
 * @subpackage  Templates.isis
 *
 * @copyright   Copyright (C) 2005 - 2014 Open Source Matters, Inc. All rights reserved.
 * @license     GNU General Public License version 2 or later; see LICENSE.txt
 */

defined('_JEXEC') or die;

jimport('joomla.filesystem.file');

class JFormFieldCompile extends JFormField
{
    protected function getInput()
    {

        $app         = JFactory::getApplication();
        $template    = $app->getTemplate(true);
        $params      = $template->params;
        $jinput      = $app->input;
        $compile     = 0;
        $compile     = $jinput->get('compileless');
        $currentpath = dirname(realpath(__DIR__));
        $pageurl     = str_replace('&amp;compileless=1', '', JURI::getInstance());

        if ($compile)
        {
            // Load the RAD layer to use its LESS compiler
            if (!defined('FOF_INCLUDED'))
            {
                require_once JPATH_LIBRARIES . '/fof/include.php';
            }

            $less = new FOFLess;
            $less->setFormatter(new FOFLessFormatterJoomla);

            //update variables.less
            $less->setVariables(array(
                "templateColor"    => $params->get('templateColor'),
                "headerBackground" => $params->get('headerColor'),
                "sidebarColor"     => $params->get('sidebarColor')
            ));

            try
            {
                $less->compileFile($currentpath . '/less/template.less', $currentpath . '/css/template.css');
            } catch (Exception $e)
            {
                $app->enqueueMessage($e->getMessage(), 'error');
            }
        }

        return '<button onclick="window.location.href=\'' . $pageurl . '\'+\'&amp;compileless=1\'" class="btn btn-primary" type="button">Compile Less</button>';
    }

}

Visiting the admin -> templates -> isis [style] you should see the compile button. Press it an you will recompile the template.css