changed input placeholder color with lighten

build/scss/_bootstrap-variables.scss

@@ -410,7 +410,7 @@ $input-focus-color:                     $input-color !default;
 $input-focus-width:                     0 !default;
 $input-focus-box-shadow:                none !default;
 
-$input-placeholder-color:               $gray-600 !default;
+$input-placeholder-color:               lighten($gray-600, 15%) !default;
 
 $input-height-border:                   $input-border-width * 2 !default;
 

dist/css/adminlte.css

@@ -1904,27 +1904,27 @@ pre code {
 }
 
 .form-control::-webkit-input-placeholder {
-  color: #6c757d;
+  color: #939ba2;
   opacity: 1;
 }
 
 .form-control::-moz-placeholder {
-  color: #6c757d;
+  color: #939ba2;
   opacity: 1;
 }
 
 .form-control:-ms-input-placeholder {
-  color: #6c757d;
+  color: #939ba2;
   opacity: 1;
 }
 
 .form-control::-ms-input-placeholder {
-  color: #6c757d;
+  color: #939ba2;
   opacity: 1;
 }
 
 .form-control::placeholder {
-  color: #6c757d;
+  color: #939ba2;
   opacity: 1;
 }