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.new {
    margin-right: 8px;
    font-size: 15px;
}

Install and activate this customized Custom Genesis Featured Posts Widget plugin: http://d.pr/f/r5GB

/**
 * Function to check if a post is older than certain days
 *
 * @param integer $days Number of days.
 * @return True if post is older than the supplied number of days, otherwise false
 * @link http://wpengineer.com/1913/if-post-is-older-than/
 */
function is_old_post( $days = 5 ) {
    $days = (int) $days;
    $offset = $days * 24 * 60 * 60;
    if ( get_post_time() < date( 'U' ) - $offset ) {
        return true;
    }

    return false;
}

add_filter( 'genesis_post_title_output', 'custom_post_title_output' );
/**
 * Filter Genesis Post Titles to add a [NEW] label for posts that have been published within the last 90 days.
 *
 * @param string $output Current title output markup.
 */
function custom_post_title_output( $output ) {

    // if this is a Post and is NOT older than 90 days i.e., was published in the last 90 days.
    if ( 'post' === get_post_type() && ! is_old_post( 90 ) ) {
        $output = '<span class="new alignleft">[NEW]</span>' . $output;
    }

    return $output;

}