Make sub navigation background color darker for better visual separation

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

app/assets/stylesheets/framework/variables.scss

@@ -16,7 +16,7 @@ $border-color:          #e5e5e5;
 $focus-border-color:    #3aabf0;
 $table-border-color:    #f0f0f0;
 $background-color:      #fafafa;
-$dark-background-color: #f7f7f7;
+$dark-background-color: #f5f5f5;
 $table-text-gray:       #8f8f8f;
 
 /*