Never seen this one before, but I would start by double checking the version setting on virtual directory or website to make sure it is targeting the correct runtime.
Try this:
http://arjansworld.blogspot.com/2006/11/aspnet-20-tip-theme-is-not-member-of.html
Can you put the HTML code up here to let us take a look? Are you sure you cleaned all old DLL's out of the directory you deployed to?
I figured my problem here. I had a class named "Invoice" on my dbml as well as a page named "Invoice.aspx". This is a WebSite Project as opposed to a WebApplication so .net was correct in saying "Compiler Error Message: BC30456: 'Theme' is not a member of 'ASP.invoice_aspx'." blah blah. I wonder why this doesn't show up locally. This site was working fine on my local box, this was showing up after publishing the site to a server. Anyways, this is now working. I just renamed the aspx page and repubbed. <~~if thats a word. lol
Thanks, ~ck in San Diego