Nope... not at all.
You can edit some of the items in a template, such as a still image or a piece of footage, of course, but not all templates are accessible to the end user. Some are set as .pox files, and you need Apple's own developer software to edit these... us mere mortals can't open them.
If you apply a template to your DVDSP project then look in the assets tab you'll see a folder in there wiht the content that makes up the template. Control click on the content and choose 'reveal in finder' to get to it. Make a copy of anything you want to change, in case you need to revert!