module class suffix not working with sparky menu

But I do not want my menu buttons stretched across the page, is there any way to make the menu background stretch across the width of the page, but the buttons to stay justified to the left as they are now?

I was thinking of using a module suffix background-color but using joomla's example, it does not work with sparky what is the correct way to achieve this?

Many Thanks

You can use:

.mp_<menu position name> .menu{
....
}

css path to additionally style your menu in template_css.css