Portrait Auto Flow will add new pages which will include all of the portrait photos. Since they are new pages they will have an automatic background from your current theme. You can change the backgrounds on all portrait pages.
1. All Portrait Auto Flow pages are bordered in red.
2. Click on the page spread so that it enlarges for editing.
3. Click on the Background tab located on the lower left side of the page spread.
4. You can use a background on both pages or just the left or right side.
5. Click on the Type dropdown to see the background options: Border, Color, Pattern, Photo, and Template. Select the type of background you want to use and click on Change.