image menu with this plugin

i use this code to add image in left or right of menu item
but not work

body.custom div#art-hmenu ul.art-hmenu li {
padding-right: 30px;
padding-top: 3px;
padding-bottom: 3px;
}

body.custom div#art-hmenu .menu-item-72 {
background: url(images/contactus.png);
background-repeat: no-repeat;
background-position: right center;
}

i want add this to custom.css but custom.css not exist
i use “simple custom css” plugin
but not worked
please help me

https://www.ads-software.com/plugins/simple-custom-css/