fixes #4957: use input border var for select element instead of #ddd

docs/assets/css/bootstrap.css

@@ -1073,7 +1073,7 @@ input[type="file"] {
 select {
   width: 220px;
   background-color: #ffffff;
-  border: 1px solid #bbb;
+  border: 1px solid #cccccc;
 }
 
 select[multiple],

less/forms.less

@@ -162,7 +162,7 @@ input[type="file"] {
 // Make select elements obey height by applying a border
 select {
   width: 220px; // default input width + 10px of padding that doesn't get applied
-  border: 1px solid #bbb;
+  border: 1px solid @inputBorder;
   background-color: @inputBackground; // Chrome on Linux and Mobile Safari need background-color
 }