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.navbar-default .navbar-nav>li>a{
	font-weight: bold;
	font-size: 1.5em;
	color: transparent;
  -webkit-background-clip: text;
}

/*===== Inside Loop=====*/
 <?php $feat_image_url = wp_get_attachment_url( get_post_thumbnail_id() );?>
  <style>
    .navbar-default .navbar-nav>li>a{
      background-image: url('<?php echo $feat_image_url;?>');
    }
  </style>