I've done a little more experimentation & Cavan is correct about which image WB is using - Toolbars/Toolbar Background.
It also appears that the image vertically "spans" the three rows in Explorer (the "menubar" which is really a toolbar, the Toolbar and the Address bar). It would also appear that the top 2-3 pixel rows of the image get used to fill the Folder pane header, stretching them to do so. I'm going to try something tricky - use a solid 2-3 pixel border at the top of the toolbar background image & see if I'm correct. Not sure how to make both look right at the same time, yet, however. While on the subject, I'd much rather WB6 fixed these sorts of issues than added blur to ppx borders. 0.02