Customize Editor Styles

@dalbeab already answered your question, but thought I would point out a way to add horizontal rule to your editor if you wish.

Within functions.php, you can add this:

// add horizontal rule button
function enable_more_buttons($buttons) {
    $buttons[] = 'hr';
    return $buttons;
}
add_filter('mce_buttons', 'enable_more_buttons');

Then you will have a TinyMCE button that looks like this which will let you add an <hr /> with only 1 click: