From Joomla! Documentation
佈景主題 controls the overall look and layout of your site and how your site would appear to people. 它提供了一個框架來將一些元素結合在一起，也就是 模組 以及 元件 ，也包括網站的 CSS。在您網站的前台 (Site) 以及 後台 (Administrator) 都分別有其佈景主題。
When Joomla! is installed several templates are automatically included. You can find many more templates at other websites or create your own according to Joomla! standards. Some are available without charge under various licenses, and some are for sale. In addition, there are many designers available who can make custom templates. You can also make your own template.
Templates are managed with the Template Manager. You will also use the Template Manager for Switching templates. To use the Template Manager, log in to the Back-end (Administrator) of your site.