So you have selected your photos, right? Well, then that should have insight as to what your theme is.
If the photos are of your kids or family – what are they doing in the photos? You can actually have several themes in one book, but there should one common thing. For instance, you can have photos of your children (the common subject) but they are doing different activities (different themes) in one scrapbook.
You can also have a common theme – such as Christmas and have different family members or friends over the years at Christmas time in one Scrapbook.
The options are endless and only limited by your creativity.