Not sure what you're wanting to tell me with your above posted link. I found this statement in one of the FileMaker links regarding custome themes.
"While you can modify layouts, it is not possible in FileMaker Pro 12 to modify and save a theme. We realize that you may want to create custom themes to use within your databases or to share with others. We may consider such a feature in the future."
I don't want to beleve this - surely FM doesn't expect data designers/developers to be trapped inside of such a limitation as only 40 very similar themes.
How do I sustome add 15 navigation buttons to apperar on a theme?
For years, many of us have set up one layout with Nav buttons, sample fields, etc and then used duplicate layout to make copies of the first layout. You then use Layout Setup... to point the layout to the desired table occurrence in Manage | Database | Relationships. The sample fields on this layout can then be duplicated and each can in turn be pointed at the correct table occurrence and field.
It's far from perfect and does not enable you to:
create a template so that changes to template can be done once instead of all 60 layouts in the db
You have to make such changes to each an every layout, but it does help maintain a certain level of interface design consistency between layouts.