I've merged Bartosz's patch (https://gerrit.wikimedia.org/r/#/c/82828/
for rolling out the boxes to all forms, not just vforms.
He used the same colors as the vform styles. It seems to me that the
success and error boxes might be better with a little more color contrast.
You can test on Beta (http://en.wikipedia.beta.wmflabs.org/
) by editing
your preferences (success), going to Special:UserLogin while logged in
(warning), and entering the wrong password for login (error).