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/* Accessibility */

/* Text meant only for screen readers */
.says,
.screen-reader-text {
	clip: rect(1px, 1px, 1px, 1px);
	height: 1px;
	overflow: hidden;
	position: absolute !important;
	width: 1px;
	/* many screen reader and browser combinations announce broken words as they would appear visually */
	word-wrap: normal !important;
}


/* post navigation */

.post-navigation {
	clear: both;
	margin: 0 -5px 0;
	overflow: hidden;
	padding: 0 5px 60px;
	border-top:1px solid #ececed;
}

.post-navigation .nav-links > div{
	width:47%;
	float:left;
	background: none !important;
}

.post-navigation .nav-links > .nav-next{
	float:right;
	text-align:left;
}

.post-navigation a {
	color: #1a1a1a;
	display: block;
	padding: 25px 0 0;
}


.post-navigation .meta-nav {
	display: block;	
	font-size: 12px;
	font-weight: 500;
	text-transform: uppercase;
	line-height: 1.6153846154;
	margin-bottom: 0.5384615385em;
}

.post-navigation .nav-next .meta-nav, .post-navigation .nav-links > .nav-next{
	text-align: right;
}

.post-navigation .meta-nav .left_arrow, .post-navigation .meta-nav .right_arrow{
	position: relative;
	vertical-align: middle;
	top:-3px;
}

.post-navigation .post-title {
	display: inline;
	font-size: 18px;
	line-height: 1.21;
	text-rendering: optimizeLegibility;
	color: #00a1e0;
}

.post-navigation a:hover .post-title,
.post-navigation a:focus .post-title {
	color: #000;
}

@media only screen and (max-width: 599px){
	.post-navigation .nav-links > div {
	  float: none;
	  width: 100%;
	}

	.post-navigation .nav-next .meta-nav, .post-navigation .nav-links > .nav-next{
		text-align: left;
	}
}

<?php 

	// Previous/next post navigation.
	the_post_navigation( array(
		'next_text' => '<span class="meta-nav" aria-hidden="true">' . __( 'Next <span class="right_arrow bigger"></span>', 'twentyfifteen' ) . '</span> ' .
			'<span class="screen-reader-text">' . __( 'Next post:', 'twentyfifteen' ) . '</span> ' .
			'<span class="post-title">%title</span>',
		'prev_text' => '<span class="meta-nav" aria-hidden="true">' . __( '<span class="left_arrow bigger"></span> Previous', 'twentyfifteen' ) . '</span> ' .
			'<span class="screen-reader-text">' . __( 'Previous post:', 'twentyfifteen' ) . '</span> ' .
			'<span class="post-title">%title</span>',
	) );

?>