I am currently working on a website I didn't code in the first place. It was created by someone else on Wordpress using the Sidney Theme and there is no child-theme.
Here is my problem : when I update the header.php file by going to Appearance -> Editor, the website crashes and all the CSS disappears.
Fortunalely, I made a backup of the website so I recovered it.
I never had any problem like this one. It is the first time I post on Stack Overflow so if you need any info just tell me.
The website is under version 5.1.1 of Wordpress.
Here is header.php file if it can help :
<?php
/**
* The header for our theme.
*
* Displays all of the <head> section and everything up till <div
id="content">
*
* @package Sydney
*/
?><!DOCTYPE html>
<html <?php language_attributes(); ?>>
<head>
<meta charset="<?php bloginfo( 'charset' ); ?>">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="profile" href="http://gmpg.org/xfn/11">
<link rel="pingback" href="<?php bloginfo( 'pingback_url' ); ?>">
<?php if ( ! function_exists( 'has_site_icon' ) || ! has_site_icon()
) : ?>
<?php if ( get_theme_mod('site_favicon') ) : ?>
<link rel="shortcut icon" href="<?php echo
esc_url(get_theme_mod('site_favicon')); ?>" />
<?php endif; ?>
<?php endif; ?>
<?php wp_head(); ?>
</head>
<body <?php body_class(); ?>>
<?php do_action('sydney_before_site'); //Hooked: sydney_preloader() ?>
<div id="page" class="hfeed site">
<a class="skip-link screen-reader-text" href="#content"><?php _e( 'Skip to content', 'sydney' ); ?></a>
<?php do_action('sydney_before_header'); //Hooked: sydney_header_clone() ?>
<header id="masthead" class="site-header" role="banner">
<div class="header-wrap">
<div class="container">
<div class="row">
<div class="col-md-4 col-sm-8 col-xs-12">
<?php if ( get_theme_mod('site_logo') ) : ?>
<a href="<?php echo esc_url( home_url( '/' ) ); ?>" title="<?php bloginfo('name'); ?>"><img class="site-logo" src="<?php echo esc_url(get_theme_mod('site_logo')); ?>" alt="<?php bloginfo('name'); ?>" /></a>
<?php else : ?>
<h1 class="site-title"><a href="<?php echo esc_url( home_url( '/' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a></h1>
<h2 class="site-description"><?php bloginfo( 'description' ); ?></h2>
<?php endif; ?>
</div>
<div class="col-md-8 col-sm-4 col-xs-12">
<div class="btn-menu"></div>
<nav id="mainnav" class="mainnav" role="navigation">
<?php wp_nav_menu( array( 'theme_location' => 'primary', 'fallback_cb' => 'sydney_menu_fallback' ) ); ?>
</nav><!-- #site-navigation -->
</div>
</div>
</div>
</div>
</header><!-- #masthead -->
<?php do_action('sydney_after_header'); ?>
<div class="sydney-hero-area">
<?php sydney_slider_template(); ?>
<div class="header-image">
<?php sydney_header_overlay(); ?>
<img class="header-inner" src="<?php header_image(); ?>" width="<?php echo esc_attr( get_custom_header()->width ); ?>" alt="<?php bloginfo('name'); ?>" title="<?php bloginfo('name'); ?>">
</div>
<?php sydney_header_video(); ?>
<?php do_action('sydney_inside_hero'); ?>
</div>
<?php do_action('sydney_after_hero'); ?>
<div id="content" class="page-wrap">
<div class="container content-wrapper">
<div class="row">
