Temanizin loop kismini sununla degistirip istediginiz gibi oynayabilirsiniz:

<ul class="grid-wrap">

	<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>

	
		<li class="grid-item">

			
			<p>
				<?php if ( has_post_thumbnail() ) : ?>
                        		<a href="<?php the_permalink(); ?>"><?php the_post_thumbnail( 'large' ); ?></a>
				<?php endif; ?>
			</p>

			
			<h2 class="title"><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>

			
			<p class="author-text">Posted on <?php echo the_time('F jS, Y');?> by <?php the_author_posts_link(); ?> </p>            
		
			<?php the_excerpt(); ?>

			<?php endwhile; ?>
			<?php endif; ?>


		</li>

	<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>></div>

</ul>

Style.css'ye de bunu ekleyin:

.grid-wrap {
	display: flex;
	flex-flow: row wrap;	
	align-items: center;	
	justify-content: center;
	list-style: none;
}

.grid-item {
	flex-basis: 400px;	
	height: 500px;		
	padding: 10px;	
	margin: 10px;	
}


.grid-item img {
	display: block;
	margin-left: auto;
	margin-right: auto;
	max-width: 400px;
}

.grid-wrap ul {
	margin-left: 0;
}