Your template itself contains 2 backgrounds, for the first slide and the common slide. Unfortunately, PowerPoint by default supports only 2 slide masters. The third slide in the template is the special printing slide, it is a file named print.jpg in your download. You can use it to create a common slide.
To set it as a background do the following:
- Open the presentation.
- Click on the slide with the right mouse button.
- Choose “Format Background”.
- Select “Picture” tab, then “File”.
- Specify picture and click “Apply”.