I shall look into that*. In theory it shouldn't be a massive amount of effort to fix; but in saying that I've just guaranteed that it will be. :|
Edit: * when I am suitably awake to alter the template in question, that is. CSS and I do not always get along.
Thanks!

I meant *padding.
In common.css, line 87, this change should do it:
#texteditor{width:100%;padding: 2px;}