adding a semi-transparent box background between background and page content

Hi,
I need to add a semi-transparent box in a grey colour between the background parallax and the white text/buttons in each of the three sections
- Create
- Discover
- Forum
Screenshot - http://take.ms/tOo4B

Support access is ACTIVE until January 1, 2016 12:12 am
Domain - http://tinyurl.com/of3qvfv

Example done in a different website - http://tinyurl.com/oozkg29 screenshot - http://take.ms/Snhal

Thanks

Tom