In this area, you can choose from a number of already existing themes or templates for your Presentation. To choose a theme that is not the default theme for the Presentation, simply scroll down and click on "Select" for the Theme that you wish to use.
Click on 'View Presentation' in the gray bar above to see what your Presentation now looks like.
While you may choose to use one of the Themes that are available for your Presentation, you may also edit the Themes to make them more personalized. To do so, click on the icon to the right of the Theme that you've selected to use (hovering over it, you will see "Edit Styles").
Once you've decided to edit the Theme, you'll be able to change a great deal of the Presentation. Simply click on the area you wish to modify in the left-hand menu, and begin making changes.
Note: when you make changes here, you are only changing the Theme for this individual Presentation! The Theme will still be available in its default form for other Presentations.