The navigation bar at the top demonstrates a full width menu that aligns the main menu items to the right.

Custom CSS: 

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#nav .menu {
    margin: 0 0 0 auto !Important;
}

Similarly, if you want to use the popup menu widget to display a full width menu that aligns the main menu items to the right, use the CSS class option of the widget to add a custom class to it (e.g. right_aligned_menu) and use this class instead of #nav in the style.css of your child theme: 
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.right_aligned_menu .menu {
    margin: 0 0 0 auto !Important;
}