I would not use Filemaker 12 on Os X 10.12. Nor 13 or 14, FWIW.
Did you open these files with subsequent versions of Filemaker, maybe doing Theme-related work on the layouts ?
FileMaker 12 will work under 10.12 - although I would not recommend that. Might be a theme Problem, maybe font problem
The so called "FileMaker 12 - Disaster"!
Be aware that FM12 hit every records in FMI history with a amount of almost 400 confirmed Bugs!
Dozens of them related to Rendering processes.
I recommend to go with FMPA 16. It's stable, you have great improvement when building layouts thanks to the new "LayoutObjectsWindow".
Be cautious with the new functions such as JSON, Tableau and REST API.
FM 11 works well from 10.9 to 10.11.6 and randomly with 10.12 in general. It plants from time to time in execution.
FM 12, 13, 14, 15 and 16 work perfectly with 10.12.6. I had some problems with Applescript, other than that, no anomalies found outside known bugs.
Getting black windows in Sierra / Filemaker 13 is quite common among my clients. Resizing / moving them will eventually reveal the data.
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Open the layout and select the change themes menu item.
Change the theme for the layout to one of the new themes. This may require adjustment.
Duplicate the db and open the duplicate
Select a field. Change text to black. Open Inspector and select second from right tabl
Choose Save for both definition and theme. This might work. If it does, do the same in the original.
Best is to change to a newer theme since the old ones are being cast aside.
The Change Themes is not available on the custom database. But it did work on the Starter Solutions db.