jQuery UI Dialog - missing close icon

From a functioning modal, the icon looks like its coming from jQuery.ui

In my example: core/assets/vendor/jquery.ui/themes/base/images/ui-icons_777777_256x240.png

Do you have custom css on the site where it doesn't work for any of the selectors:

.ui-button .ui-icon {
    background-image: url(images/ui-icons_777777_256x240.png) /*{iconsDefault}*/;
}

.ui-widget-header .ui-icon {
    background-image: url(images/ui-icons_444444_256x240.png) /*{iconsHeader}*/;
}

.ui-button-icon-only .ui-icon {
    position: absolute;
    top: 50%;
    left: 50%;
    margin-top: -8px;
    margin-left: -8px;
}

.ui-icon-closethick {
    background-position: -96px -128px;
}

?