The best way to do this is use a Child Theme. You only need two files: style.css and comments.php.
This way is best because it doesn’t modify any of the original code used in the Pinboard theme. If you upgrade the parent theme, your previous and next post links should still work.
style.css:
/*
Theme Name: Pinboard Child
Description: Child theme for Pinboard
Template: pinboard
Version: 0.1.0
*/
@import url("../pinboard/style.css");
.single .entry { margin-bottom: 10px !important; }
.prevnext_post { margin: 0 2.5% 5.1%; }
.prevnext_post >div {
-webkit-box-sizing: border-box;
-moz-box-sizing: border-box;
box-sizing: border-box;
width: 50%;
}
.prevnext_post .prev { float: left; padding-right: 10px; }
.prevnext_post .next { float: right; padding-left: 10px; text-align: right; }
comments.php:
<div class="prevnext_post">
<div class="prev"><?php previous_post_link('%link','← %title'); ?></div>
<div class="next"><?php next_post_link('%link','%title →') ?></div>
<div class="clear"></div>
</div>
<?php include(get_template_directory().'/comments.php'); ?>