Pure-CSS / YUI Vertical multi-level menu

Question

There are a few issues about the styles for that vertical nav that make it awkward (the overflow-y had to be removed to allow any sub nav to display).

<!doctype html>  
<html lang="en">  
<head>  

<!-- This page uses PURE CSS. See documentation at http://purecss.io/ -->
<meta charset="utf-8">  
<meta http-equiv="X-UA-Compatible" content="IE=edge">

<meta name="viewport" content="width=device-width, initial-scale=1.0">  

<!-- For HTML5, place favicon.ico and apple-touch-icon(s) in the root directory -->

<!--Add PURE Base, which essentially is built-in Normalize.css, 
    an HTML5-ready CSS reset with minimal styling. -->
<link rel="stylesheet" href="http://yui.yahooapis.com/pure/0.4.2/pure-min.css">

<!--Add yui for second-level menu support. -->
<script src="http://yui.yahooapis.com/3.14.1/build/yui/yui-min.js"></script>    
<style>
body {
    color: #777;
}

.pure-img-responsive {
    max-width: 100%;
    height: auto;
}

/*
Add transition to containers so they can push in and out.
*/
#layout,
#menu,
.menu-link {
    -webkit-transition: all 0.2s ease-out;
    -moz-transition: all 0.2s ease-out;
    -ms-transition: all 0.2s ease-out;
    -o-transition: all 0.2s ease-out;
    transition: all 0.2s ease-out;
}

/*
This is the parent `<div>` that contains the menu and the content area.
*/
#layout {
    position: relative;
    padding-left: 0;
}
    #layout.active {
        position: relative;
        left: 150px;
    }
        #layout.active #menu {
            left: 150px;
            width: 150px;
        }

        #layout.active .menu-link {
            left: 150px;
        }
/*
The content `<div>` is where all your content goes.
*/
.content {
    margin: 0 auto;
    padding: 0 2em;
    max-width: 800px;
    margin-bottom: 50px;
    line-height: 1.6em;
}

.header {
     margin: 0;
     color: #333;
     text-align: center;
     padding: 2.5em 2em 0;
     border-bottom: 1px solid #eee;
 }
    .header h1 {
        margin: 0.2em 0;
        font-size: 3em;
        font-weight: 300;
    }
     .header h2 {
        font-weight: 300;
        color: #ccc;
        padding: 0;
        margin-top: 0;
    }

.content-subhead {
    margin: 50px 0 20px 0;
    font-weight: 300;
    color: #888;
}



/*
The `#menu` `<div>` is the parent `<div>` that contains the `.pure-menu` that
appears on the left side of the page.
*/

#menu {
    margin-left: -150px; /* "#menu" width */
    width: 150px;
    position: fixed;
    top: 0;
    left: 0;
    bottom: 0;
    z-index: 1000; /* so the menu or its navicon stays above all content */
    background: #191818;
    -webkit-overflow-scrolling: touch;
}
    /*
    All anchors inside the menu should be styled like this.
    */
    #menu a {
        color: #999;
        border: none;
        padding: 0.6em 0 0.6em 0.6em;
    }

    /*
    Remove all background/borders, since we are applying them to #menu.
    */
     #menu .pure-menu,
     #menu .pure-menu ul {
        border: none;
        background: transparent;
    }

    /*
    Add that light border to separate items into groups.
    */
    #menu .pure-menu ul,
    #menu .pure-menu .menu-item-divided {
        border-top: 1px solid #333;
    }
        /*
        Change color of the anchor links on hover/focus.
        */
        #menu .pure-menu li a:hover,
        #menu .pure-menu li a:focus {
            background: #333;
        }

    /*
    This styles the selected menu item `<li>`.
    */
    #menu .pure-menu-selected,
    #menu .pure-menu-heading {
        background: #1f8dd6;
    }
        /*
        This styles a link within a selected menu item `<li>`.
        */
        #menu .pure-menu-selected a {
            color: #fff;
        }

    /*
    This styles the menu heading.
    */
    #menu .pure-menu-heading {
        font-size: 110%;
        color: #fff;
        margin: 0;
    }

#menu .pure-menu-children {
    background: #191818;
}

/* -- Dynamic Button For Responsive Menu -------------------------------------*/

/*
The button to open/close the Menu is custom-made and not part of Pure. Here's
how it works:
*/

/*
`.menu-link` represents the responsive menu toggle that shows/hides on
small screens.
*/
.menu-link {
    position: fixed;
    display: block; /* show this only on small screens */
    top: 0;
    left: 0; /* "#menu width" */
    background: #000;
    background: rgba(0,0,0,0.7);
    font-size: 10px; /* change this value to increase/decrease button size */
    z-index: 10;
    width: 2em;
    height: auto;
    padding: 2.1em 1.6em;
}

    .menu-link:hover,
    .menu-link:focus {
        background: #000;
    }

    .menu-link span {
        position: relative;
        display: block;
    }

    .menu-link span,
    .menu-link span:before,
    .menu-link span:after {
        background-color: #fff;
        width: 100%;
        height: 0.2em;
    }

        .menu-link span:before,
        .menu-link span:after {
            position: absolute;
            margin-top: -0.6em;
            content: " ";
        }

        .menu-link span:after {
            margin-top: 0.6em;
        }


/* -- Responsive Styles (Media Queries) ------------------------------------- */

/*
Hides the menu at `48em`, but modify this based on your app's needs.
*/
@media (min-width: 48em) {

    .header,
    .content {
        padding-left: 2em;
        padding-right: 2em;
    }

    #layout {
        padding-left: 150px; /* left col width "#menu" */
        left: 0;
    }
    #menu {
        left: 150px;
    }

    .menu-link {
        position: fixed;
        left: 150px;
        display: none;
    }

    #layout.active .menu-link {
        left: 150px;
    }
}

</style>

</head>  
<body>  

<div id="layout">
    <!-- Menu toggle -->
    <a href="#menu" id="menuLink" class="menu-link">
        <!-- Hamburger icon -->
        <span></span>
    </a>

    <div id="menu">
        <div class="pure-menu pure-menu-open">
            <a class="pure-menu-heading" href="#">Company</a>

            <ul>
                <li><a href="#">Home</a></li>
                <li><a href="#">About</a></li>

                <li class="menu-item-divided pure-menu-selected">
                    <a href="#">Services</a>

                        <ul>
                            <li class="pure-menu-heading">Sub Nav Heading</li>
                            <li class="pure-menu-separator"></li>
                            <li><a href="#">Sub 1</a></li>
                            <li><a href="#">Sub 2</a></li>

                            <li>
                                <a href="#">Sub 3</a>
                            </li>

                            <li><a href="#">Sub  4</a></li>
                        </ul>
                </li>

                <li><a href="#">Contact</a>
            </li>
            </ul>
        </div>
    </div>

    <div id="main">
        <div class="header">
            <h1>Page Title</h1>
            <h2>A subtitle for your page goes here</h2>
        </div>

        <div class="content">
            <h2 class="content-subhead">The Layout</h2>
            <p>
                This is the basic "Side Bar" page design I want to customize. I got it from the Pure CSS <a href="http://purecss.io/layouts/" target="_blank">Layouts page</a>.
            </p>



<!-- Script for adding drop-down. -->
<script>
YUI({
    classNamePrefix: 'pure'
}).use('gallery-sm-menu', function (Y) {

    var horizontalMenu = new Y.Menu({
        container         : '#menu',
        sourceNode        : '#menu > .pure-menu > ul',
        orientation       : 'vertical',
        hideOnOutsideClick: false,
        hideOnClick       : false
    });

    horizontalMenu.render();
    horizontalMenu.show();

});
</script>


<p>&nbsp;</p>
<p>&nbsp;</p>
<p style="color:garnet; font-weight:bold;">I am trying to learn how to add the above submenu functionality to Pure's vertical menu, which displays on the left side of this page.</p>


        </div>
    </div>
</div>

</body>
<!-- Script to make vertical menu responsive. -->
<script src="http://purecss.io/combo/1.11.5?/js/ui.js"></script>
</html>