icingaweb2/application/layouts/scripts/parts/menu.phtml

<?php
if (! $this->level) {
    $this->level = 0;
}
?>
<ul<?= $this->level === 0 ? ' role="navigation"' : '' ?>>
<?php

foreach ($this->items as $item) {

    printf(
        '  <li%s><a href="%s">%s%s</a>',
        $this->href($this->url) === $this->href($item->getUrl()) ? ' class="active"' : '',
        $item->getUrl() ? $this->href($item->getUrl()) : '#',
        $item->getIcon() ? $this->img(
            $item->getIcon(),
            array('class' => 'icon')
        ) . ' ' : '',
        $this->escape($item->getTitle())
    );

    if ($item->hasChildren()) {
        echo $this->partial(
            'parts/menu.phtml',
            array('items' => $item->getChildren(), 'url' => $this->url, 'level' => $this->level + 1)
        );
    }

    echo "</li>\n";
}

?>
</ul>