Refactor md variables

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/stylesheets/base/mixins.scss

@@ -55,10 +55,10 @@
 }
 
 @mixin md-typography {
-  color: #444;
+  color: $md-text-color;
 
   a {
-    color: #3084bb;
+    color: $md-link-color;
   }
 
   img {

app/assets/stylesheets/base/variables.scss

@@ -2,6 +2,8 @@ $hover: #FFFAF1;
 $gl-text-color: #54565b;
 $gl-header-color: #4c4e54;
 $gl-link-color: #333c48;
+$md-text-color: #444;
+$md-link-color: #3084bb;
 $nprogress-color: #c0392b;
 $gl-font-size: 15px;
 $list-font-size: 15px;