By rights, if you apply a theme using Stardock programs, it should still be on your desktop after a Win 10 update. Having said that, however, with the various consequences of Windows Update of late, I wouldn't bet the farm on it.
I blocked the latest update due to some users losing all their personal files, but the previous major update disabled or uninstalled many of my 3rd-Party programs, so who knows.
The odds are that your theme will be intact after the next update, but don't quote me on it. okay.