I am using the jQuery dialog widget with the modal option over a wordpress site.
The background (overlay) is not all grey, there is a white stripe across the background.
$("#popup").dialog({
dialogClass: "alert",
width: crmpJs.setup.social_popup_custom_content_use ? crmpJs.setup.social_popup_setup_width : 640,
height: crmpJs.setup.social_popup_custom_content_use ? crmpJs.setup.social_popup_setup_height : "auto",
autoOpen: true,
modal: true,
resizable : false,
draggable : false,
zIndex: 10000,
closeOnEscape: crmpJs.setup.social_popup_content_locker_use == "yes" ? false : true
});

Any idea of the reason for this malfunction?
It’s your CSS. Change the repeat-x to just repeat:
to: