Can themes style the visual editor?

Short answer: yes. This has been available for quite some time but don’t think I’ve seen themes that do it. Things like typefaces, headings, image padding and borders, etc. can easily be set by the current theme making the visual editor… more WYSIWYG. All it takes is a stylesheet and a small function in the theme’s functions.php file:

add_filter('mce_css', 'my_editor_style');
function my_editor_style($url) {

  if ( !empty($url) )
    $url .= ',';

  // Change the path here if using sub-directory
  $url .= trailingslashit( get_stylesheet_directory_uri() ) . 'editor-style.css';

  return $url;
}

where¬†editor-style.css will be applied to TinyMCE’s iframe.

There is also an option to define CSS classes that can be inserted by the user and to add the “Styles” drop-down selector to the editor toolbar. This is probably most useful for developers when creating a custom theme for a client as these classes would stop working if the theme is changed.

I’ve put together a small package with examples of how to enable this. If you are creating WordPress themes, download it or have a look, it’s quite simple. If not, perhaps ask the author of your theme to add it to the next update. :)

Update: as Dion points out in the comments, this is not meant for adding the whole “style.css” to the editor. That could bring problems and most of the styling won’t apply there anyways. It works best when a separate stylesheet is made specifically for use in the editor (editor-style.css in the example). It should contain a small subset from style.css, usually the part that is applied to the actual posts.