Have you tried using themes available in the admin console? You should be able to select that template from the drop down list of available templates, customize it and then set that set of template(s) as the global theme.
We cannot figure out where and how in the code the /decorator/default/template.ftl (whether the default or the theme) gets applied to the other page ftls.
There are two parts: the site gets wrapped using a technology called Sitemesh, all the decoration happens there. The theming of Freemarker files happens through a combination of a number of technologies, the core stuff being in JiveFreemarkerManager and ThemeTemplateLoader.
Here's a great screencast on theming Clearspace: http://www.jivesoftware.com/community/blogs/podcasts/2008/01/17/introduction-to-skinning-clearspace
Also, be sure to check out the release doc tutorial here, which will give you a good foundation for overriding Freemarker templates to get the look and feel you want for your site: http://www.jivesoftware.com/builds/docs/clearspace/latest/ClearspaceThemingGuide.html