Open the app's menu to see the complete list of options. Each of the themes included in Galaxy Themes is different from all the rest, and you can see what they look like by tapping on them. That way, you'll have an idea of what your smartphone will look like if you decide to use that theme.
To change your Samsung's theme, all you have to do is choose one and set it as predefined. It only takes a few seconds!
Once you close the settings, you'll see the new theme on your smartphone. To change it back, all you have to do is open Galaxy Themes again and choose the one you like best.