Regression due to MDL-63028. To replicate:

1. Add the following css to the "Raw initial SCSS" section under advanced settings in the Boost theme settings:

   body {

   background-image: url("https://hq.moodle.com/pluginfile.php/1/theme_more/backgroundimage/1480391664/Orange-highlights.jpg");

   background-size: cover;

   background-repeat: repeat;

   }

2. Save changes, purge caches and visit a course home page. Notice the background of #region-main inherits the image, and is no longer #fff.