HTML editor
If you know HTML, CSS, JS, Smarty, and you want to make advanced changes to your theme, you can do so via the HTML editor. You will find it in the 'Layout' tab.
On the left side you'll see all HTML files. Select the one you want to edit, make all modifications, and click on 'Save changes'.
Remember that if you modify the theme yourself, the theme will loose its warranty and support.