Help! Cannot center Featured Image.- New Gamer Theme

I am trying to center a Featured image on my static home page. I have tried all kinds of suggestions updating style.css, tried the Simple Custom CSS plugin, updated index.php with

<?php the_post_thumbnail(‘category-thumb’, array(‘class’ => ‘aligncenter’)); ?>

Then ran the regenerate thumbnails plugin. But nothing works. The image just keeps aligning to the left.

Can anyone help? I can’t post a link to my site as it is still in development.