After logging into CMSimple the admin menu opens at the top of the screen. Under:
You can now activate the template of your choice (Selectbox). This template now is activated globally, for all pages of your CMSimple installation.
With the "Page" tab you can open another menu, here you will find the page-specific settings, one of them is a select box with all installed templates.
The template you choose here will only be activated for the active page.