w2ui Grid: How to set custom icon on toolbar buttons?

.custom-save
{
background-image: url("https://SharePoint/../SiteAssets/save-16.png");
    background-repeat: no-repeat;
    width: 12px;
    height: 12px;
    background-position: center;
}

toolbar: {

        items: [

        { type: 'break' },

            { type: 'button', id: 'savechanges', caption: 'Save Changes', img: 'custom-save', style:'color:#0062af;font-weight:bold;'}

            

        ],