I can't say what's going on with your particular website, but in case another anecdotal datapoint will help:
In early April I refactored the HTML and applied some new CSS styling to a site that has been getting moderately high and consistent traffic from Google for the last several years. No content changes, no URL changes.
Rankings dropped instantaneously and I lost about 2/3 traffic the next day. Fortunately I didn't panic and revert my changes, because on the second day traffic was back to normal.
Over the next month, Google traffic again dropped precipitously two more times, each time regaining to normal levels a few days later.
Things seem to have stabilized, and Google traffic has remained high since May.
So, presuming that your redesign is equally easily indexed by Google, I'd guess that you might see your traffic jump up again once their algorithms are sufficiently convinced that your site hasn't been hacked by spammers. For how long now has your traffic been low?