silverstripe-subsites/templates/SilverStripe/Admin/LeftAndMain_Menu.ss

<div class="cms-mobile-menu-toggle-wrapper"></div>

<div class="fill-height cms-menu cms-panel cms-panel-layout" id="cms-menu" data-layout-type="border" aria-expanded="false">
    <div class="cms-menu__header">
        <% include SilverStripe\\Admin\\LeftAndMain_MenuLogo %>
        <% include SilverStripe\\Admin\\LeftAndMain_MenuStatus %>

        <% if $ListSubsites %>
            <% include SubsiteList %>
        <% end_if %>
    </div>

    <div class="flexbox-area-grow panel--scrollable panel--triple-toolbar cms-panel-content">
        <% include SilverStripe\\Admin\\LeftAndMain_MenuList %>
    </div>

    <div class="toolbar toolbar--south cms-panel-toggle">
        <% include SilverStripe\\Admin\\LeftAndMain_MenuToggle %>
    </div>
</div>

<button class="fill-height fill-width cms-menu-mobile-overlay" aria-controls="cms-menu" aria-expanded="false"></button>