Thank you for the post.
If even a single layout is not in the classic theme, then that error will be displayed when trying to use Instant Web Publishing. Are all the layouts in the database file set to the classic theme?
When a new database file is created, a non-classic theme is applied by default. The theme will need to be adjusted to classic for each individual layout in the file.
File>Manage>Layouts will show all the layouts in the current file. If all layouts in the database are set to classic, then test a new database with a single layout. If the issue persists on the test file, let me know and we will discuss further troubleshooting.
For additional information about Instant Web Publishing feature in FileMaker Pro 12 see Knowledge Base article 10766: