Thank you for your post.
Save a custom theme in FileMaker Pro 13. Go into Layout Mode, pull down the Layouts menu and select "Change Theme...". At the top of the listing of Themes, you will see a section "Custom" with the custom themes listed below it.
Open the file in FileMaker Pro 12. Go into Layout Mode, pull down the Layouts menu and select "Change Theme...". Notice, no custom themes are listed.
Custom Themes are a new feature and not available in FileMaker Pro 12. Therefore, if you are going to also use this in FileMaker Pro 12, use one of the built-in themes, and create a rectangle within that theme with a fill color, no border and rounded corners This will then display properly in FileMaker Pro 12.