This template was (at the time of writing) used in around 2000 pages on Wikiversity, making it probably the most frequently used template on Wikiversity. In addition, the template is key to the main page, user guides, browse page, major portals.... The best way to screw up Wikiversity is a single careless edit to this template.
So far two (well-intentioned, highly capable) custodians managed to misunderstand the template and introduce format errors. The trouble with complex templates is that sometimes only their originally creators really understand how they work and what their idiosyncracies are (perhaps because the code tends to end up being rather individualised). This is a template where you could easily go wrong, however talented you are.
Keep the main page (and a few other key pages) open in separate browser tabs, and refresh with a cache purge every time you edit this template. This may quickly make any side-effects clear to you.
Contact McCormack (i.e. me, who made the original version) on IRC/skype and try to have me hanging around in real time just in case something goes wrong.
Consider the possibility of provisionally forking or (programmatically) extending the template with your modified version, so that your modified version doesn't go live until it's been properly tested. The forked/extended version can be inserted into just a few test pages until it's clear that no damage will occur.
You can't edit it directly, but you can ask a custodian (e.g. McCormack) or you can view the source and copy it into a new template, which you can then experiment with and use.
Theme = 15 appears to create both a mauve header and body, as opposed to theme = 5, which is a mauve header and lighter body. Any theme outside of 1-15 produces a white header and body. Here's an updated chart: