Make the background and the footer gradiations customizable

This change extracts following variables for allowing customizatin:
- nav-bg-back
- nav-bg-front
- footer-bg-back
- footer-bg-front

.themes/classic/sass/base/_theme.scss

@@ -23,6 +23,8 @@ $type-border: #ddd !default;
 
 /* Navigation */
 $nav-bg: #ccc !default;
+$nav-bg-front: image-url('noise.png');
+$nav-bg-back: linear-gradient(lighten($nav-bg, 8), $nav-bg, darken($nav-bg, 11));
 $nav-color: darken($nav-bg, 38) !default;
 $nav-color-hover: darken($nav-color, 25) !default;
 $nav-placeholder: desaturate(darken($nav-bg, 10), 15) !default;
@@ -46,6 +48,8 @@ $twitter-status-link: lighten($sidebar-link-color-subdued, 15) !default;
 
 $footer-color: #888 !default;
 $footer-bg: #ccc !default;
+$footer-bg-front: image-url('noise.png');
+$footer-bg-back: linear-gradient(lighten($footer-bg, 8), $footer-bg, darken($footer-bg, 11));
 $footer-color: darken($footer-bg, 38) !default;
 $footer-color-hover: darken($footer-color, 10) !default;
 $footer-border-top: lighten($footer-bg, 15) !default;

.themes/classic/sass/custom/_colors.scss

@@ -5,9 +5,14 @@
 //$header-bg: #263347;
 //$subtitle-color: lighten($header-bg, 58);
 //$nav-bg: desaturate(lighten(#8fc17a, 18), 5);
+//$nav-bg-front: image-url('noise.png');
+//$nav-bg-back: linear-gradient(lighten($nav-bg, 8), $nav-bg, darken($nav-bg, 11));
 //$sidebar-bg: desaturate(#eceff5, 8);
 //$sidebar-link-color: saturate(#526f9a, 10);
 //$sidebar-link-color-hover: darken(#7ab662, 9);
+//$footer-bg: #ccc !default;
+//$footer-bg-front: image-url('noise.png');
+//$footer-bg-back: linear-gradient(lighten($footer-bg, 8), $footer-bg, darken($footer-
 
 
 /* To use the light Solarized highlighting theme uncomment the following line */

.themes/classic/sass/partials/_footer.scss

@@ -4,7 +4,7 @@ body > footer {
   color: $footer-color;
   text-shadow: lighten($footer-bg, 5) 0 1px;
   background-color: $footer-bg;
-  @include background(image-url('noise.png'), linear-gradient(lighten($footer-bg, 8), $footer-bg, darken($footer-bg, 11)));
+  @include background($footer-bg-front, $footer-bg-back);
   border-top: 1px solid $footer-border-top;
   position: relative;
   padding-top: 1em;

.themes/classic/sass/partials/_navigation.scss

@@ -1,7 +1,7 @@
 body > nav {
   position: relative;
   background-color: $nav-bg;
-  @include background(image-url('noise.png'), linear-gradient(lighten($nav-bg, 8), $nav-bg, darken($nav-bg, 11)));
+  @include background($nav-bg-front, $nav-bg-back);
   border: {
     top: 1px solid $nav-border-top;
     bottom: 1px solid $nav-border-bottom; }