Easiest solution: In your user preferences, change the skin, which your user account uses, to Monobook or so, essentially to anything that is not Vector. Then fix the page, e.g. by going back to an older revision and switch the skin back to Vector.
Some way more difficult solutions would be this: If you have direct access to the database, you could fix it by manipulating the conents of the Vector.css page inside the database. Apart from that it will be hard to guess what you have changed (and so what you would have to change back, e.g. using Firebug) in order to make the editing field and the button visible again.