Currently if you want to add theme.skin settings for mojoPortal modules, you must register the module at the top of the theme.skin file, but if the registration exists and the module is not installed it causes a very unfriendly error.
For instance, if you download a skin that has a special configuration for Form Wizard Pro, it will have Form Wizard Pro registered in the theme.skin like this:
<%@ Register Namespace="sts.FormWizard.Web.UI" Assembly="sts.FormWizard.Web.UI" TagPrefix="stsfw" %>
In this case, your skin will be rejected if you do not have Form Wizard Pro installed. This can be solved easily by simply commenting out this line in the theme.skin, but if you fail to do that it will break all site styling. Ideally this issue would just be logged in the system log, and not prevent the skin from working.