Add this code into your child theme’s functions.php
file:
function graphene_footer_content(){ ?>
<div class="custom-footer-content">
<!-- Put your HTML codes here -->
</div>
<?php }
add_action('graphene_developer', 'graphene_footer_content');
Make sure you replace <!-- Put your HTML codes here -->
with whatever HTML codes you want to put in there, and then position/style it using CSS.