[Theme: Twenty Twelve] Menu Seperator Bullets implemented, broke responsive

Hi – I added a separator between menu items at this little site.

When I shrink the window to show the mobile menu, it appears I’ve broken the menu by adding these bullets.

How can I repair this or add some kind of conditional that prevents it from displaying these on the mobile site?

URL: https://www.sarniapost.com/

CSS added:

#menu-main-menu li {
	padding-right: 60px;
	background: url('https://sarniapost.com/resources/menusep2.png') center right no-repeat;
}

#menu-main-menu li:last-child {
	padding-right: 0;
	background: none;
}