Cannot apply custom css to IE in the theme

Looks like TwentyFourteen includes the ie.css stylesheet within a ‘less than IE 9‘ conditional tag, which would explain why it’s not working for IE11.

<!--[if lt IE 9]>
   <link rel="stylesheet" id='twentyfourteen-ie-css'  href="http://localhost/wordpress_answers/wp-content/themes/twentyfourteen/css/ie.css" type="text/css" media="all" />
<![endif]-->

Sounds like you’ll want to enqueue your own IE specific stylesheet. This is probably done cleanest from within a child theme, so you can update 2014 when possible.

You can use this code to enqueue your own IE specific stylesheet:

// Load the Internet Explorer specific stylesheet.
wp_enqueue_style( 'my-ie-styles', get_template_directory_uri() . '/css/my_ie_styles.css', array( 'twentyfourteen-style', 'genericons' ), '20131205' );
wp_style_add_data( 'my-ie-styles', 'conditional', 'IE' );

wp_style_add_data() will output the link tag within an IE specific conditional:

<!--[if IE]>
<link rel="stylesheet" id='my-ie-styles-css'  href="http://localhost/wordpress_answers/wp-content/themes/twentyfourteen/css/my_ie_styles.css" type="text/css" media="all" />
<![endif]-->