.ui-resizable-handle 'background':

The above element with the blue border is "resizable" (as well as being draggable).

The divs used as handles for resizing can be styled using the .ui-resizable-handle class, as I've done in applying a yellow border to them.

However, they seem to resist application of background color styles -- I've also specified background-color: red for the class .ui-resizable-handle, and they remain a light gray.