In looking at the list of "FileMaker 15 removed features" I espy the "Classic theme". I confess that I've made essentially zero use of themes since they became available. I create hardly any new databases from scratch. What I do is grab stuff from older files that's close to what I want and modify it for each new use. Yes, that means that each layout object is formatted individually, and yes, I understand that this wouldn't be the best way to go about it if I were starting from scratch, but I'm not starting from scratch, and avoiding "reinventing the wheel" seems to me to be a more efficient use of my time.
I'm aware that some older features that aren't supported any more (for example, the "embossed" appearance of the bazillion buttons I've deployed) still work as originally intended, even tho you can't create them anew. But I can copy and paste them, and those "obsolete" features still go along for the ride.
My hesitation about upgrading to FMP 15 is partly based on a concern that, in the absence of support for the Classic theme, I'll get some kind of unexpected meltdown in the appearance of my legacy designs (which is to say, all of my designs). Need I be concerned?