修改placeholder颜色

.form-control::-webkit-input-placeholder { /* WebKit browsers */
  color:#ccc;
}
  
.form-control:-moz-placeholder { /* Mozilla Firefox 4 to 18 */
  color:#ccc;
}
  
.form-control::-moz-placeholder { /* Mozilla Firefox 19+ */
  color:#ccc;
}
  
.form-control:-ms-input-placeholder { /* Internet Explorer 10+ */
  color:#ccc;
}