Custom page templates are a powerful feature of WordPress that can be used to make pages (not posts) do something different from normal. You can make them do pretty much anything. Obviously, this can greatly increase the flexibility of your site.
However, because of the way most WordPress themes are created, calling an external CSS file from a custom page template is not straightforward. Here, I explain the problem, look at the options and explain how to use ‘plugin functionality’ to overcome the problem.