WordPress [provides a `mce_css` filter][1], that can be used to add custom stylesheets to the Visual Editor. According to the Codex:

> The file can be a .php file, allowing dynamic generation of CSS rules
> for the content editor.

Example Codex filter callback, modified for a Theme:

    function plugin_mce_css( $mce_css ) {
    	if ( ! empty( $mce_css ) )
    		$mce_css .= ',';
    
    	$mce_css .= get_template_directory_uri() . '/dynamic-css.php';
    
    	return $mce_css;
    }
    
    add_filter( 'mce_css', 'plugin_mce_css' );

  [1]: http://codex.wordpress.org/Plugin_API/Filter_Reference/mce_css