1. Go to your specific Live Editor.
2. Click on Layout Setting >> Header & Navigation >> make sure to empty the field of the header backround color and update.
3 After that go to Page Setting >> Other Script >> Custom CSS >> insert the below code.
background: rgba(0, 0, 0, 0.8);
Please feel free to use your own background color using RGB color
You'll have to change the RGB color and the 0.8 is the opacity which can be changed if needed. A good site to find RGB color codes is: http://www.colorpicker.com/