In your theme directory open this file: templates/loop-blog.php
Find the line 40, it looks like:
<a class="entry-categories" href="<?php echo esc_url( get_category_link( $first_cat->term_id ) ); ?>"><?php echo esc_html( $first_cat->name ); ?></a>
Replace it by:
<a class="entry-categories category-<?php echo $first_cat->term_id; ?>" href="<?php echo esc_url( get_category_link( $first_cat->term_id ) ); ?>"><?php echo esc_html( $first_cat->name ); ?></a>