I just ran into this problem myself. Here is the link a colleague sent me regarding this issue: Instant Web Publishing not supported in FM12 without Classic Theme?
The only problem I have, is that I have followed the steps to change to Classic theme, as I'm in a hurry to get my db up, and can worry about the specific formatting changes to make it more eye appealing later, but I STILL get the message that my db cannot be displayed via IWP because it doesn't use the classic theme. I have made sure (three times!) that all of my layouts are in classic theme, and still get the same error message. So, if you use the steps in this link and figure out a way around this issue (if you even encounter it) please share the solution!
Make a copy of your database.
Open Manage | database | Relationships in the copy
Double Click each and every "box" in this window and select "Add FileMaker Data Source" to "repoint" every table occurrence to the matching table in the original copy of the file.
Go to Tables and delete all the tables. You now have an interface file with all your original layouts/themes.
Go to your original copy of the file and change the themes of each and every layout to Classic. Edit your layouts as needed so that they look decent in Classic theme.
Publish your original copy of the file to the web. Have your FileMaker clients connect to the new copy of the file that still contains the originally themed layouts.