"If you had created a style for your journal, you would still be able to use that style for your journal layout. However, you wouldn't be able to modify that style in any way, save for the few pointers given in the FAQs. See ( http://www.livejournal.com/support/faqbrowse.bml?faqcat=customize ) for information on customization."
Add something about keeping mood themes, but not being able to edit them.
I also think it should be added that you can switch between your custom ones and the system ones while a free user without losing them.