When you are in the book creation part, just click on the Template button, in the taskbar at the top left of your book.
You will then see the already chosen theme with all its layouts, and by clicking on the double arrow Change theme.
The app will display all the Categories (wedding, anniversary, family...). You just have to click on one of them to access the different themes of the category.
Once the category chosen, by clicking on a theme you will have an overview of the layouts to help you choose, and to confirm click on the Choose these models button.
Then you will simply click on the layout of your choice for it to be applied to the page of your photo album. You can change the theme as many times as you wish.