Semi-transparent background for content

You can use rgba or opacity declaration for background-color

for example

div { background-color: rgba(255,0,0,1); }

or

div { background-color: rgb(255,0,0); opacity: 1; }

You need to link the website in question if you want a specific CSS example

I just gave you the example.
You need to declare your choice of color and opacity. Opacity can be anything between 0 to 1 (i.e. 0.5 for 50% transparency)

Hi ZOOM1881,

You need to add Custom CSS for that and the way to add that is
Go to Appearance -> Theme Options -> Design -> Custom CSS
And paste the below code and if you want more transparent than you can change the opacity 0.8 to your desired and 255 color numbers as your desired

#page{
  background: rgba(255, 255, 255, 0.8);
}

Regards
chhabi