how to make a back-ground semi-transparent

Hello, 
I'm using your theme Movie with Muffin Builder. 
I would like to know how it is possible to make the background semi-transparent ? 
for example on this page http://www.lasupplication-lefilm.com/svletana/, I would like the gray color to be semi-transparent so we can see the picture behind, and the text still is readable. 
Thanks a lot for your advise

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