Here, we add the Bootstrap container class to get the responsive grid system applied to our site.

wp-content/themes/customtheme/index.php

<div class="container">
    <?php if ( have_posts() ) : ?>
        <?php while ( have_posts() ) : the_post(); ?>
            <h2>A single blog post.</h2>
            <?php the_title(); ?>
            <?php the_content(); ?>
        <?php endwhile; ?>
    <?php endif; ?>
</div>