Randomize posts problem

Hello, I have trouble randomizing my posts. I have googled all around and looked for plugins, but don’t know how to fix this.

I used to get this working with the following code:

[please read http://codex.ww.wp.xz.cn/Forum_Welcome#Posting_Code for posting code]

<?php query_posts($query_string . '&orderby=rand') ?>

But with the theme I’m now using it won’t work, I don’t really know why…

The index.php shows this:

<?php get_header(); ?>
			<?php get_sidebar('top'); ?>
			<?php
			if (have_posts()) {
				/* Display navigation to next/previous pages when applicable */
				if (theme_get_option('theme_' . (theme_is_home() ? 'home_' : '') . 'top_posts_navigation')) {
					theme_page_navigation();
				}
				/* Start the Loop */
				while (have_posts()) {
					the_post();
					get_template_part('content', get_post_format());
				}
				/* Display navigation to next/previous pages when applicable */
				if (theme_get_option('theme_bottom_posts_navigation')) {
					theme_page_navigation();
				}
			} else {
				theme_404_content();
			}
			?>
			<?php get_sidebar('bottom'); ?>
<?php get_footer(); ?>

What would be the best way to randomize it with this index.php

Many thanks!!