As crauscher said, Office does not seem to trigger any public events when the theme user for the 2007 Office application changes.
The simplest thing you could do is regularly poll the registry key to see if a change has occurred:
Office 2007: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Common\Theme
Office 2010: HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Common\Theme
Where is the theme 1for blue, 2for Silver and 3for black scheme.
#, : BackColor Office 2007