I'm writing my blog post in ReST code, so naturally I want that the HTML edition tab of wordpress have a fixed-width font as monospace. Here's how to achieve this.

Go edit your own themes file wp-content/themes/mytheme/functions.php, and add these lines:

...
add_action( 'admin_print_styles-post.php', 'my_admin_css' );
add_action( 'admin_print_styles-post-new.php', 'my_admin_css' );
function my_admin_css() {
     ?>
     <style type="text/css">
     #editorcontainer textarea#content { font-family: monospace!important }
     </style>
     <?php
}
...

It's done !

Thanks a lot to WordPress forum...