I used an ftp client and edited the bit of .css from Tim above into my skin.css, for the skin I am actually using, appending it at the end of the file.  It worked for me. 

I am hosted on PoweDNN, who did my upgrade for me, but who could only refer me to the ftp means of editing my .css file.  So I bore down and gave it a try confident I could undo it it if failed.  The site had been at 5.6.3 before this and I still feel there's a bit missing from understanding why it happened and if it is completely skin dependent (I think so -- a couple of my sites using very basic and very old skins didn't have this issue).  I'd suggest the Core Team look at this as 6.x evolves...I'd hate to see a bunch of folks with workarounds for an easy 'corporate community' problem lead to snags down line because the workaround gets left in and forgotten. Or a skin change in the future causes it to recur with a different webmaster with no background.  Or, ......