@tdkuehnel: The important part of the traceback you are seeing above is the last line:
RequirementParseError: Invalid requirement, parse error at "'/ploneth". This error is telling you that buildout thinks that you've asked it to install a new egg (and you have), but that the "requirement" (which really means the name of the egg, possibly combined with a version pin) is invalid.
Buildout believes that the string
"'/ploneth" is all or part of an egg you've asked it to install. My guess is that there is something wrong with the way in which you added the name of your new theme package to the eggs list in the buildout.cfg file. I guess this because that's what you've changed recently, and also because generated theme packages usually start with the namespace "plonetheme".
Egg "requirements" should look like this:
Notice that there are no path separators in this line of text, nor are there any quotation marks. This line should be only the name of the package you are trying to add to your buildout (the name of your new theme package).
Find the place where you've added "/ploneth" in your buildout.cfg file and fix it. This error will go away.