Hi Greg,

Looks like the office folks already figured this out out. I found the
following blog from Reghavendra Prabhu describing the necessity of  
creating an activating an activation context:

 
http://blogs.gotdotnet.com/rprabhu/PermaLink.aspx/6b1a5495-c790-4e2f-972f-af
b01f0164cf

I was going to write up a sample myself to do this for another customer,
but found the following KB article, which beat me to it. It applies to
Office addin's. But I suspect the same technique will work with your addin
as well. You just need to use that using(new EnableThemingInScope) trick
around the creation of your control or dialog.

  http://support.microsoft.com/default.aspx?scid=kb;en-us;830033

Sincerely,
Ed Dore
Microsoft Developer Support
(425)704-2048
mailto:eddo@microsoft.com