Background image defined at:

packages/enlil_transparent_yogurt/themes/enlil_transparent_yogurt/images/default.jpg

Background style at:

packages/enlil_transparent_yogurt/themes/enlil_transparent_yogurt/elements/header.php

The main container background transparency color and opacity level can be changed by editing this line in packages/enlil_transparent_yogurt/themes/enlil_transparent_yogurt/main.css just below the "main styles" section:

background: rgba(0, 0, 0, 0.7);

There are a few minor changes to the main.css and typography.css files to accomplish widening the content areas and evening out the box shadow around the main content area, etc.