Improve index.php template

<?php

/*

    Theme: Fachschaft Computerlinguistik

    Index Template File

/**

 * Template for the index page.

 * 

 * WordPress identifies this file by its name (which MUST be index.php)

 * and displays it whenever it has to display lists of posts. Furthermore,

 * this page is used as the ultimate fallback page when there is no

 * other template available.

 * 

 * @see https://codex.wordpress.org/Theme_Development#Index_.28index.php.29

 */

get_header();

?>

<aside>

    <div class="sidebar-content">

        <h1><div class="square-icon icon-news"></div>

        <h1>

            <div class="square-icon icon-news"></div>

            <?php

                if ( is_category() ) {

                    single_cat_title();

</aside>

<main class="list">

    <?php

        while ( have_posts() ) : the_post();

        ?>

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

        <article>

            <?php if ( has_post_thumbnail($post->ID) ) : ?>

            <div class="post-header" style="background-image:url('<?php echo wp_get_attachment_url( get_post_thumbnail_id($post->ID) ); ?>');">