Add missing, and assumed, white background for screen rendering.

[SVN r43337]

doc/html/boostbook.css

@@ -372,6 +372,10 @@
 
     @media screen
     {
+        body {
+            background-color: #FFFFFF;
+        }
+        
     /* Links */
         a
         {

doc/src/boostbook.css

@@ -372,6 +372,10 @@
 
     @media screen
     {
+        body {
+            background-color: #FFFFFF;
+        }
+        
     /* Links */
         a
         {