One thing I noticed with an Anonymous Blog site in Foundation is, the user always gets the default theme. I did some googling today and ran across some info in the comments section of this bloghttp://www.novolocus.com/2010/04/14/themes-in-sharepoint-2010/
I followed the steps below in my v4.master page and it works for me. 🙂 I noticed that the calendar dates were still using the default css and not using the theme, but adding the blog.css link resolve it for me.
You will need to copy this link and put it into your master page – I put mine below the tag. SharePoint:SPHelpPageComponent