simplify themes by using less

also adding additional css rules to less which have already been in the assets/css but not in less (base.less, responsive.less)

assets/less/default/responsive.less

@@ -1,6 +1,10 @@
 /* Extra small devices (phones, less than 768px) */
 @media (max-width: 767px) { 
     
+    body{ 
+        padding: 0;
+    }
+    
     .sidebar-wrapper {
         position: static;
         width: inherit;