Hi Paul,
However you must be aware of the following:
Exceptions raised due to routing-matching problems will not be theme-aware, because the logic used to determinate whether an user is viewing a backend (URLs with `admin` prefix) or frontend page depends on the current request object (which is build by Cake's Router). So if an URL doesnt matches any valid route is impossible to build a valid request object and then impossible to decide if backend or frontend theme should be used. In this cases the frontend theme will be used by default.
You could also try to use site-wide template feature, you must place your template files at `ROOT/templates/Common`.
Hope this helps,
Best regards!