The code that generates the tweet button is outdated and you’ll have to manually update it if you want it to display properly.
In functions.php find this block of code:
<a href="<?php echo esc_url( 'https://twitter.com/share' ); ?>" class="twitter-share-button" data-url="<?php the_permalink(); ?>">Tweet</a>
<script>!function(d,s,id){var js,fjs=d.getElementsByTagName(s)[0];if(!d.getElementById(id)){js=d.createElement(s);js.id=id;js.src="//platform.twitter.com/widgets.js";fjs.parentNode.insertBefore(js,fjs);}}(document,"script","twitter-wjs");</script>
Replace it with this:
<a href="https://twitter.com/share" class="twitter-share-button" data-url="<?php the_permalink(); ?>" data-show-count="true">Tweet</a><script async src="//platform.twitter.com/widgets.js" charset="utf-8"></script>