имя_скина.php

$R['form_guest_remember'] = '<input type="checkbox" name="rremember" class="checkmark" />';
$R['form_guest_remember_forced'] = '<input type="checkbox" name="rremember" checked="checked" disabled="disabled" class="checkmark" />';

в login.tpl    блок должен быть именно label

<div class="flex_block">
	<label class="loging_checkbox">{USERS_AUTH_REMEMBER} 
		<div class="checkmark"></div>
	</label>
	<div class="center">{PHP.L.users_rememberme}</div>
</div>

CSS

.loging_checkbox input {
  position: absolute;
  opacity: 0;
  cursor: pointer;
  height: 0;
  width: 0;
  left: -5px;
 }
 
 .loging_checkbox input.checkmark
 {
  width:35px; 
 }

 .loging_checkbox {
  display: inherit;
  position: relative;
  cursor: pointer;
  font-size: 20px;
  user-select: none;
  order:2;
 }
 
 /* Create a custom checkbox */
 .checkmark {
  position: relative;
  top: 0;
  height: 1.3em;
  width: 1.3em;
  background: #98a47b;
  border-radius: 5px;
 }
 
 /* When the checkbox is checked, add a blue background */
 /*
 .loging_checkbox input:checked ~ .checkmark {
  background-image: linear-gradient(#b9e9b3,#a8e4a0)
 }
 */
 /* Create the checkmark/indicator (hidden when not checked) */
 .checkmark:after {
  content: "";
  position: absolute;
  display: none;
 }
 
 /* Show the checkmark when checked */
 .loging_checkbox input:checked ~ .checkmark:after {
  display: block;
 }
 
 /* Style the checkmark/indicator */
 .loging_checkbox .checkmark:after {
  left: 0.45em;
  top: 0.25em;
  width: 0.25em;
  height: 0.5em;
  border: solid #58634d;
  border-width: 0 0.15em 0.15em 0;
  transform: rotate(45deg);
 }

 .center
 {
  vertical-align: middle;
 }