Christmas decorations are probably not an article of faith, so if your boss can't be persuaded to change her mind, there may be little you can do about it.
However, if it's an office where members of the public have access, you could try arguing that Christmas decorations are good for business.
If Christmas decorations are put up, be safety conscious. Use non-flammable items and don't have them hanging in areas where they might be a fire risk. Oh, and don't stand on a chair which is on a desk, just so you can get the mistletoe in the centre of the room – or it may be more than a heart that is broken this Christmas.