restore margins on thumbnails, add comments about it's changes and how responsive affects it

less/close.less

@@ -23,7 +23,7 @@
 button.close {
   padding: 0;
   cursor: pointer;
-  background-color: transparent;
+  background: transparent;
   border: 0;
   -webkit-appearance: none;
 }

less/responsive-1200px-min.less

@@ -16,6 +16,9 @@
   .thumbnails {
     margin-left: -30px;
   }
+  .thumbnails > li {
+    margin-left: 30px;
+  }
   .row-fluid .thumbnails {
     margin-left: 0;
   }

less/responsive-768px-979px.less

@@ -12,4 +12,6 @@
   // Input grid
   #grid > .input(42px, 20px);
 
+  // No need to reset .thumbnails here since it's the same @gridGutterWidth
+
 }

less/thumbnails.less

@@ -1,5 +1,6 @@
 // THUMBNAILS
 // ----------
+// Note: `.thumbnails` and `.thumbnails > li` are overriden in responsive files
 
 // Make wrapper ul behave like the grid
 .thumbnails {
@@ -16,6 +17,7 @@
 .thumbnails > li {
   float: left; // Explicity set the float since we don't require .span* classes
   margin-bottom: @baseLineHeight;
+  margin-left: @gridGutterWidth;
 }
 
 // The actual thumbnail (can be `a` or `div`)