How to add {less} files to Omega HTML5 subtheme?
Install the LESS module and then in your custom-html5.info file just add your LESS CSS files like you would normally add stylesheets:-

stylesheets[all][] = css/custom-html5-alpha-default.less


The module will detect the LESS files (those with a .less file extension) and process them so that they are output as standard CSS.

Processing the LESS files on the server rather than via the browser using JavaScript will give the end user a better experience and ensures that the look of your site isn't dependent on JavaScript. If you really must include the less.js file then perhaps you could consider using the drupal_add_js() method in your template.php file.