Render 'screen' element the same way as 'programlisting'.

[SVN r22502]

doc/html/boostbook.css

@@ -21,7 +21,7 @@ th
    margin-bottom: 0pc;
 }
 
-div pre.programlisting, .refsynopsisdiv
+div pre.programlisting, .refsynopsisdiv, pre.screen
 {
    margin-left:  3pc;
    margin-right: 3pc;
@@ -66,7 +66,7 @@ div pre.programlisting, .refsynopsisdiv
       background-color: #DDD;
    }
 
-   .programlisting, .table-programlisting
+   .programlisting, .table-programlisting, .screen
    {
       background-color: #EED;
       border:           0.2em ridge #EED;