Thanks for that. I modified it slightly:

textarea {
    width:  64em;
    height: 32em;
}
[download]

That looks fine for me to start with - I'll probably play with it as time goes on.

I had picked a Theme but didn't know I could add CSS to that - the field in Display Settings looked like replacement CSS.

I don't use the sidebar while composing a post and losing the overhead it incurs would be a good thing.