clean up variable usage with colors

lib/carousel.less

@@ -70,7 +70,7 @@
   line-height: 30px;
   color: #ccc;
   text-align: center;
-  background: #999;
+  background: @grayLight;
   background: rgba(0,0,0,.5);
   .border-radius(20px);
 
@@ -82,9 +82,9 @@
 
   // Hover state
   &:hover {
-    color: #fff;
+    color: @white;
     text-decoration: none;
-    background: #333;
+    background: @grayDark;
     background: rgba(0,0,0,.75);
   }
 }
@@ -98,10 +98,10 @@
   right: 0;
   bottom: 0;
   padding: 10px 15px 5px;
-  background: #333;
+  background: @grayDark;
   background: rgba(0,0,0,.75);
 }
 .carousel-caption h4,
 .carousel-caption p {
-  color: #fff;
+  color: @white;
 }