The only skin we ship with mojoPortal is "framework". This is primarily to keep the release file smaller in size.
If something in the layout.master causes problems with the master file from being rendered, it will fallback to a very plain no-frills skin that still lets you access administration so you can change the skin or look at the log. Unfortunately, if there is a problem in the theme.skin the skin will crash the site. It's always been this way but for a long time mojoPortal didn't use the theme.skin much and none of the shipped skins had anything in the theme.skin for the pro modules.
We use the theme.skin a lot because we can use special settings on controls in the theme.skin to control the markup and classes used throughout the site and on specific features. We are looking for ways to trap errors thrown by the theme.skin but we haven't come up with anything yet.