The themes on the Add-ons site are for the toolbar area of the browser. What web page(s) do you want to theme? If the site(s) do not have that feature, you would need to get those from a different kind of add-on. Or if you mean built-in pages, you may need to apply custom style rules to those pages (this may require some additional graphics because the toolbar theme graphics are not very tall, so simply injecting them into the page may not look very good).