In order to select a ready made theme, click the green 'New' button at the top of the page. Select the Standard Template or Mobile Template. You will see a range of thumbnails of all ready to use themes where you can either click to Preview or Select. This will take you to the editing page.
Once a theme has been activated/selected, it will add to your available themes in the grid. You can switch between available themes by clicking the edit button within the image container.
Select a theme