Lately I've noticed some of my newly compiled, personal GUI apps open up with a XP theme even though I have an embedded Windows 10 manifest. After wasting my time looking for a complicated reason and solution, I discovered that Windows was setting these new apps to Windows 95 compatibility mode. Thank you Microsoft ...
The fix is simply to turn off compatibility for those .exe's in the properties dialog.
Hopefully this saves someone from chasing their tail.
Windows 10 Pro (64-bit) Version 2004 Build 19041.450
Windows Feature Experience Pack 120.2212.31.0