This is actually caused by a typo in their code. Its an easy fix. Open /include/staff/profile.inc.php and edit line 557, from
<input type="radio" id="dark_mode_radio" name="dark_mode" class="switch nullable" <? echo $staff->getExtraAttr("dark_mode", "false") == "on" ? "checked='checked'" : "" ?>>
to
<input type="radio" id="dark_mode_radio" name="dark_mode" class="switch nullable" <?php echo $staff->getExtraAttr("dark_mode", "false") == "on" ? "checked='checked'" : "" ?>>
(They are missing the “php” tag before the echo)