WordPress Template CSS .archive

Hello I am currently studying WordPress template by a textbook. And I want to set background color #fff on each articles from archives. but I am thinking if the textbook sample CSS is correct or not because it won't work if I don't comment out ".archive" what is ".archive" for on this css?

This is textbook sample link. And every user try to learn by making it step by step. http://pacific2.prime-strategy.co.jp/archives/category/column/

archive.php <?php get_header(); ?> <section id="contents"> <header class="page-header"> <h1 class="page-title"><?php single_cat_title(); ?></h1> </header> <div class="posts"> <?php if (have_posts()) : while (have_posts()) : the_post(); get_template_part('content-archive'); endwhile; endif; ?> </div> </section><!-- #contents end --> <?php get_sidebar(); ?> <?php get_footer(); ?>

content-archive.php <article <?php post_class(); ?>> <a href="<?php the_permalink(); ?>"> <?php the_post_thumbnail('large_thumbnail', array('alt'=> the_title_attribute('echo=0'), 'title'=> the_title_attribute('echo=0'))); ?> </a> <header class="entry-header"> <time pubdate="pubdate" datetime="<?php the_time('Y-m-d'); ?>" class=" entry-date"><?php the_time(get_option('date_format')); ?></time> <h1 class="entry-title"><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h1> </header> <section class="entry-content"> <?php the_excerpt(); ?> </section> </article>

css /* column / / .archive */ article.hentry, .search article.hentry { background-color: #fff; padding: 20px; border-top: solid 1px #ddd; border-bottom: solid 1px #ddd; }

/* .archive */ article.hentry .attachment-large_thumbnail, .search article.hentry .attachment-large_thumbnail { background-color: #fff; padding: 4px; border: solid 1px #ddd; float: left; }

/* .archive / article.hentry .entry-header, / .archive */ article.hentry .entry-content, .search article.hentry .entry-header, .search article.hentry .entry-content { margin-left: 150px; }

/* .archive */ article.hentry time, .search article.hentry time { position: relative; top: -4px; }

span.author { font-size: 80%; display: block; float: right; }

/* .archive */ article h1, .search article h1 { font-size: 150%; font-weight: normal; line-height: 1; margin-bottom: 5px; }