Click on the Theme tab. There are several themes preloaded into the application and you can change to any of them at anytime. The green arrow shows you your current theme. You can preview any content you have added in a different theme by clicking the page with a magnifying glass noted by the blue arrow. This will give you a real time preview of your presentation with that theme but DOES NOT select it to be the new current theme of your presentation. To do this you must hit the "Select" box noted by the red arrow.
There are several ways you can customize your theme and presentation. This will be addressed in later pages.