SUMMARY
Global Themes that expect Plasma to set the Desktop Type to org.kde.desktopcontainment when applying their layout do not have the desktop type get changed from the user's current desktop type (usually Folder View) to it.

STEPS TO REPRODUCE
1. Make a Global Theme with the following in its 'defaults' file:

[Desktop][org.kde.plasma.desktop]
Containment=org.kde.desktopcontainment

2. Select the newly constructed Global Theme and tick 'Use desktop layout from theme'
3. Hit 'Apply'

OBSERVED RESULT
The desktop type remains as Folder View, even though the desktop background got reset meaning the desktop was very likely otherwise reset to defaults.

EXPECTED RESULT
The desktop type changes to Desktop (Desktop Containment) instead of being Folder View after applying the layout of this Look & Feel.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: KDE neon 5.17.2 (Feren OS Next 2019.11 Pre-RC)
(available in About System)
KDE Plasma Version: 5.17.2
KDE Frameworks Version: 5.63.0
Qt Version: 5.13.1

ADDITIONAL INFORMATION
Applying the Look & Feel, terminating plasmashell, deleting plasma-org.kde.plasma.desktop-appletsrc and then reloading plasmashell will result in the correct desktop type being used by Plasma.