PHP, WP, function.php: Conditional style.css register in function.php
<?php
wp_enqueue_style('ie7-fixes', get_bloginfo('template_url') . '/css/ie7.css', false, '', 'screen');
global $wp_styles;
$wp_styles->add_data('ie7-fixes', 'conditional', 'lte IE 7');
?>
alternativa:
wp_enqueue_style(
'ie7-style',
get_template_directory_uri() . '/style-ie.css'
);
global $wp_styles;
$wp_styles->add_data( 'ie7-style', 'conditional', 'lte IE 9' );