Include your custom pages in Mybb

[.m.]

^ you can try adding a css class to the rules page link - like below

<li><a href="{$mybb->settings['bburl']}/rules.php" class="ruleslink"><img src ="images/icons/lightning.png">Rules</a></li>

and also add css style property in global.css stylesheet to remove search icon for the link - like below

#header ul.menu li a.ruleslink {background-image: none!important;}