fixed a bunch of variables

lms/static/sass/base/_variables.scss

@@ -62,7 +62,7 @@ $text-color: $dark-gray;
 
 $body-bg: rgb(250,250,250);
 $header-image: linear-gradient(-90deg, rgba(255,255,255, 1), rgba(230,230,230, 0.9));
-$header-bg: transparent;
+$header-bg: rgb(255, 255, 255);
 $courseware-header-image: linear-gradient(top, #fff, #eee);
 $courseware-header-bg: transparent;
 $footer-bg: transparent;

lms/static/sass/course/layout/_courseware_header.scss

@@ -64,7 +64,7 @@ header.global.slim {
   @include box-shadow(0 1px 2px rgba(0, 0, 0, .1));
   height: 50px;
   border-bottom: 1px solid $outer-border-color;
-  background: $white;
+  background: $header-bg;
 
   .guest .secondary {
     margin-right: 0;

lms/static/sass/shared/_header.scss

 header.global {
   border-bottom: 1px solid $m-gray;
   @include box-shadow(0 1px 5px 0 rgba(0,0,0, 0.1));
-  background: $white;
+  background: $header-bg;
   height: 76px;
   position: relative;
   width: 100%;