With a bit of HTML and CSS knowledge, you can customize how your Gallery theme appears. Add some PHP and Smarty Template knowledge and you can change a theme’s functionality or create a new theme.
Before beginning to edit or create a theme, get familiar with Gallery 2’s theme elements. Theme developers should also be familiar with the Smarty template engine.
Note: Be sure to disable template caching when editing templates (.tpl files). Else you need to clear your template cache to see the change taking effect. Site Admin -> Performance - The template cache was introduced in Gallery 2.3.