Monotype font for Wordpress "Write Post" text area

I like to use a monotype font (e.g. Courier) when I edit my blog posts in HTML mode. Wordpress seems to default to a proportional font. Try as I might, I couldn't find an easy way to change this from the WP admin pages, so I resorted to hacking the responsible stylesheet, /wp-admin/wp-admin.css, adding the font-family declaration for the #content text area:

editorcontainer #content {
    font-family: Courier New, Courier, sans-serif;
    padding: 0;
    line-height: 150%;
    border: 0 none;
    outline: none;
}