Please do not confuse USING, CONFIGURING and STYLING 3rd party extension!
Nothing (and certainly not a template) is preventing you to USE JPro Final Countdown module, if you have installed it correctly and assigned it to correct template position and menu item(s).
You can also CONFIGURE it as per varous available parameters in module's configuration back ...
To change the background you want:
If you haven't already, create empty file custom.css in folder /templates/purity_iii/css/ then add to it following selector and adjust it to your liking:
background: url('your_image_path') no-repeat scroll 0 0 transparent;
Regarding that YouTube video:
You are already looking at ...