Did you try the new twitter and didn't like it?
Do you wan to stick to the old twitter and want to get rid of that annoying bar telling you "Pssst... the new version of Twitter is here. Try it now!" or "You will automatically be upgraded to New Twitter very, very soon. Switch to New Twitter now!"?
Just use this simple style.
It hides that top bar and reposition the whole background to avoid an empty space.

Update: Hide the 'Wonder who is this?' floating box next to retweets.