Try: Define the minimum boilerplate code for a block-based theme

emptytheme/assets/alignments-front.css

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+/*
+ * Alignments, loaded in the front-end only.
+ */
+
+body {
+	margin: 0;
+}
+
+* {
+	box-sizing: border-box;
+}
+
+.wp-site-blocks,
+.wp-block-template-part.alignfull {
+	padding: 0 var(--wp--custom--margin--horizontal);
+}
+
+.wp-site-blocks > *:not(.wp-block-post-content),
+.wp-site-blocks .wp-block-post-content > * {
+	max-width: var(--wp--custom--width--default);
+	margin-left: auto;
+	margin-right: auto;
+}
+
+.wp-site-blocks .alignwide {
+	width: var(--wp--custom--width--wide);
+	max-width: 100%;
+	margin-left: auto;
+	margin-right: auto;
+}
+
+.wp-site-blocks .alignfull {
+	transform: translateX(calc(0px - var(--wp--custom--margin--horizontal)));
+	width: calc(100% + (2 * var(--wp--custom--margin--horizontal)));
+	max-width: calc(100% + (2 * var(--wp--custom--margin--horizontal)));
+	margin-left: 0;
+	margin-right: 0;
+	box-sizing: content-box;
+}
+
+.wp-site-blocks .wp-block-template-part.alignfull {
+	width: 100%;
+	max-width: 100%;
+}
+
+.wp-site-blocks .wp-block-columns.alignfull {
+	width: 100%;
+	max-width: 100%;
+}
+
+.aligncenter {
+	text-align: center;
+}
+
+.wp-site-blocks .alignleft {
+	float: left;
+	margin-right: 2em;
+	max-width: 360px;
+}
+
+.wp-site-blocks .alignright {
+	float: right;
+	margin-left: 2em;
+	max-width: 360px;
+}
+
+@media screen and (min-width: 1290px) {
+
+	.wp-site-blocks,
+	.wp-block-template-part.alignfull {
+		padding: 0;
+	}
+
+	.wp-site-blocks .alignfull {
+		transform: translateX(0px);
+		width: 100% + var(--wp--custom--margin--horizontal));
+		max-width: calc(100% + var(--wp--custom--margin--horizontal));
+	}
+}

emptytheme/experimental-theme.json

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+{
+	"global": {
+		"settings": {
+			"color": {
+				"gradients": [ ],
+				"palette": [ ]
+			},
+			"typography": {
+				"customLineHeight": true,
+				"fontFamilies": [ ],
+				"fontSizes": [ ],
+				"spacing": {
+					"customPadding": true
+				}
+			},
+			"custom": {
+				"width": {
+					"default": "840px",
+					"wide": "1100px"
+				},
+				"margin": {
+					"horizontal": "14px"
+				}
+			}
+		},
+		"styles": {
+			"color": { },
+			"typography": { }
+		}
+	}
+}

emptytheme/functions.php

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+<?php
+
+if ( ! function_exists( 'emptytheme_support' ) ) :
+	function emptytheme_support()  {
+
+		// Adding support for featured images.
+		add_theme_support( 'post-thumbnails' );
+
+		// Adding support for alignwide and alignfull classes in the block editor.
+		add_theme_support( 'align-wide' );
+
+		// Adding support for core block visual styles.
+		add_theme_support( 'wp-block-styles' );
+
+		// Adding support for responsive embedded content.
+		add_theme_support( 'responsive-embeds' );
+
+		// Add support for experimental link color control.
+		add_theme_support( 'experimental-link-color' );
+
+		// Add support for editor styles.
+		add_theme_support( 'editor-styles' );
+
+		// Enqueue editor styles.
+		add_editor_style( 'style.css' );
+    }
+    add_action( 'after_setup_theme', 'emptytheme_support' );
+endif;
+
+/**
+ * Enqueue scripts and styles.
+ */
+function emptytheme_scripts() {
+
+	// Enqueue theme stylesheet.
+	wp_enqueue_style( 'emptytheme-style', get_template_directory_uri() . '/style.css', array(), wp_get_theme()->get( 'Version' ) );
+
+	// Enqueue alignments stylesheet.
+	wp_enqueue_style( 'emptytheme-alignments-style', get_template_directory_uri() . '/assets/alignments-front.css', array(), wp_get_theme()->get( 'Version' ) );
+}
+add_action( 'wp_enqueue_scripts', 'emptytheme_scripts' );

emptytheme/style.css

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+/*
+Theme Name: Empty Theme
+Theme URI: https://github.com/wordpress/theme-experiments/
+Author: Kjell Reigstad
+Description: The base for a block-based theme.
+Requires at least: 5.3
+Tested up to: 5.5
+Requires PHP: 5.6
+Version: 1.0
+License: GNU General Public License v2 or later
+License URI: LICENSE
+Text Domain: emptytheme
+
+Empty Theme is distributed under the terms of the GNU GPL.
+*/