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If you look at the menu on your left, you'll see a link to the Theme Library. You can download any of these, unzip and then upload the unzipped folder to your site via FTP to the /themes folder of your XOOPS install. Once uploaded, you will have to go to your admin menu>system>preferences>general settings, and click on the themes you want to use in the "selectable themes" section. You'll have to hold the "Control" button on your keyboard down to select multiple themes. Also, you'll need to make the Themes block visible to be able to change themes on your front page.
You probably won't need custom templates, unless the theme you choose says it needs a custom template.