override-wp-block-width.php

<?php
add_action( 'admin_enqueue_scripts', function () {
	if ( get_option( 'page_on_front' ) === filter_input(  INPUT_GET, 'post' ) ) {
		wp_add_inline_style(
			'wp-edit-post',
			'
		.block-editor-writing-flow { --block-width: 1136px; }
		.wp-block { max-width: var(--block-width); }
		'
		);
	} else {
		wp_add_inline_style(
			'wp-edit-post',
			'
		.wp-block { max-width: 812px }
		.post-type-post .editor-styles-wrapper { background-color: #fff }
		'
		);
	}

} );