[FAFO] Drop superfluous background-color of header and footer

These are not needed since the page already has the same background
color.  Remove them so they don't interfere when we put an image in the
background.

css/area_footer.less

@@ -35,7 +35,6 @@
         .main-footer {
             position: relative;
             box-sizing: border-box;
-            background-color: @ini_background_site;
             margin-top: 5px; // for box-shadow of content
             text-align: center;
         }