I really don't see the need for this and this to me is different than the Search FIelds in the BrowseDialog.  Really a users themed preferences should not be saved on a per machine basis but rather by a per logged in user basis.  If a user goes from machine to machine then the theme should follow that user.  If a different user logs into your application on the same machine, then that users preferred theme should be loaded.  We have a preferences file in our medical software and allow the user to choose their theme.  After the user logs in we then load that theme from the preferences table.  This is something that can be done very easily on your part and really may change from environment to environment based on a developers needs.

The BrowseDialogs search fields collection is a more complicated element and is internal to the BrowseDialog and there is no easy way for you, the developer, to handle this yourself other than through the use of a registry key.  There is also quite a bit more data that is saved through the SearchFields collection rather than a single shared property on the ApplicationTheme level.

Hope this makes sense.