From the documentation:
This theme is also useful as a starting point to write custom skins, particularly if they are not styleable. This is because the skin classes in the Wireframe theme file are simpler than their Spark counterparts.
So I want to use the Wireframe theme to create a custom theme, but where do I get the mxml files from?
Found it:
flex sdk 4.1\frameworks\projects\wireframe\src\spark\skins\wireframe