If you are using Windows I recommend using 7-Zip to make the tar.gz. http://www.7-zip.org/ .
Before you submit the theme check that your code matches the HTML and CSS guidelines
Then you can upload the tar.gz file to the themes directory - http://www.silverstripe.com/themes/edit and then we will check over the theme and approve/disapprove the theme.
A couple hints I suggest before you submit the theme that catches a few people out -
* make sure you have all the typography styles (h1-h6, links, lists, paragraphs, etc)
* you have support for all the main features (menu 1, sub menu, sidebar for widgets)
* Theme tested (and pretty much spot on) in at least IE6, IE7, Firefox 2.
* You are not using any copyrighted Icons or images in the theme.
* The theme follows the coding conventions (html, js).
If those aren't issues then the chance we will disapprove the theme is pretty low. I suggest from just looking at the template on your site that you make the header image / logo text based rather then a image. Just so people just swap the title out easily and hassle free. And the grey text is a bit light for me on my screen.