So you’ve got Gallery installed and now you want to use it. The first thing to understand is that in Gallery, you can create different albums for your picture collection. And just like going to a store and buying albums based on their looks (i.e., an album with a green leather cover, or a gingham and floral cloth-bound album), you can create Gallery albums to have a certain look—or theme. To control theme settings throughout Gallery, you must be an administrator or have administration privileges.
Before you can utilize a theme you need to install it. Themes are treated as plugins - therefore in your Site Admin go to Gallery >> Plugins and install the themes you like. Only the theme Matrix is installed by default.
There are several places in Gallery you’ll have to visit to control all aspects of your album themes. The first place to go is under Options: Edit Album. Under Edit Album, you’ll notice there are four tabs: General, Album, Theme, Custom Themes.