Wordpress template with fixed sidebar in flex

PHP

<?php /* Template Name: Template Sidebar */ ?>

<?php get_header(); ?>

<style>
.main-content {
    flex: 1;
    display: flex;
}	
.fixed-left-nav {
    background: #FAF9F8;
    flex: 0 0 256px;
    order: 0;
    border-right: 1px solid #d9d9d9;
}
.right-main-content {
    background: #F3F3F5;
    flex: 1 1 100px;
    order: 1;
    overflow-y: auto; 
}
</style>

<div class="main-content">
    <div class="fixed-left-nav">
      nav
    </div>
    <div class="right-main-content">
		<?php if (have_posts()): while (have_posts()): the_post(); ?>
		<h1 class="page-title"><?php the_title(); ?></h1>
		<?php the_content(); ?>
		<?php endwhile; endif; ?>		
    </div>
</div>


<?php get_footer(); ?>