If you done it, default template will not show. Some users already have cookie set with style defauult, that is why it is showing. To force all users to black, replace include/class.css.php with

Code:
<?php

class css
{

    public static function set($css)
    {
        css::validate($css);

        $redirect_url = '';

        if (isset($_SERVER['HTTP_REFERER']))
        {
            $redirect_url = $_SERVER['HTTP_REFERER'];
        }

        if (stripos($redirect_url, VSHARE_URL) === false)
        {
            $redirect_url = VSHARE_URL . '/index.php';
        }

        redirect($redirect_url);
    }

    public static function validate($css)
    {
	$css = 'black';
        $_SESSION['CSS'] = $css;
        setcookie('CSS', $css, time() + 2592000); /* expire in 30 days */
    }

    public static function cookie()
    {
        $css = 'black';
        css::validate($css);
    }
}