Cannot remove theme(s), or create layouts without one (IWP)
Operating system version
OS X 10.6.8
Description of the issue
Developers could be hindered by not being able to get rid of applied theme(s) without losing custom layout work (colors, etc.). This is more of a 'beware' issue than a bug, although perhaps some warnings could be put into effect to save folks from trouble. I am new to 12, and I could be not well learned, but here goes.
A developer has spent (a great deal of) time developing custom-designed layouts in 11 or previous, for a file in which IWP is enabled. The file is migrated to 11, and IWP works fine. The developer, who has not read all about 12, sets a theme, proceeds in development, then when testing in IWP, finds out that the theme (if not Classic), is not supported in IWP. The developer switches to the Classic theme, and her customizations are lost.
It would be really helpful to receive notice of this potential problem when applying/changing a theme, and even better yet if one could remove all themes. A pre-12-created file without a theme can work in IWP, so getting a file back to a non-themed state would be very helpful.
Steps to reproduce the problem
Build layout(s) in pre-12, for an IWP-enabled file.
Convert to 12.
Mistakenly apply a non-classic theme.
Continue development in FM.
Later test in IWP.
Find out your theme won't work.
Mandatorily switch to the Classic theme.
All your hard work is gone (unless you did proper dev. backups, and tracked your work).
Be warned of applying non Classic themes for IWP use when attempting to apply one.
Be able to remove all themes.