I just ran into this issue. Took me a few minutes to figure out. And there just isn't enough on google for this particular compile time error, so I thought I would help (searching with "The control type" and "cannot be themed" only gets nine hits presently).

If you ever create a custom control and try to theme it, you might get the following exception:

The control type 'YourNamespace.YourControlName' cannot be themed.

Add the following attribute to your class:

[Themeable(true)]

Problem solved.