Hi Richard,
There is no DisableThemes( ) system function. However, you can create a custom theme where you define "drawing":false for every visual object type.
I have attached such a "none" theme. Unzip into your "theme190" folder so the filename becomes
...\theme190\none\none.json
Now, you can turn themes off by calling
ApplyTheme("none")
The theme feature is running but takes no action on any visual object type.
HTH /Michael