You can change the available theme at any time while working on your project without changing the content you have already created. To do so, please click on Theme at the top of your Mixbook editor, next to the Page Tools button. A new window will appear, and you can select which theme you'd like to change your book to by clicking on the desired theme.
Please note that following these steps will change the available elements and layouts, but will not change any pages currently in your project. To add pages from the new theme to your project, we would recommend highlighting a current page that you do not want or creating a new page, then selecting a layout from your new theme from the "Layouts" tab on the upper left side of the screen.