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

blockbase/assets/alignments-editor.css

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+/*
+ * Alignments, loaded in the editor only.
+ */
+
+.wp-block {
+	max-width: var(--wp--custom--width--default);
+}
+
+.wp-block[data-align="wide"] {
+	max-width: var(--wp--custom--width--wide);
+}
+
+.wp-block[data-align="full"] {
+	max-width: none;
+}

blockbase/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--width-horizontal)));
+	width: calc(100% + (2 * var(--wp--custom--width-horizontal)));
+	max-width: calc(100% + (2 * var(--wp--custom--width-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--width-horizontal));
+		max-width: calc(100% + var(--wp--custom--width-horizontal));
+	}
+}

blockbase/experimental-theme.json

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+{
+	"global": {
+		"settings": {
+			"color": {
+				"gradients": [ ],
+				"palette": [ ]
+			},
+			"typography": {
+				"customFontSize": true,
+				"customLineHeight": true,
+				"fontSizes": [ ],
+				"spacing": {
+					"customPadding": true,
+					"units": [ "px", "em", "rem", "vh", "vw" ]
+				}
+			},
+			"custom": {
+				"width": {
+					"default": "580px",
+					"wide": "1100px"
+				},
+				"margin": {
+					"horizontal": "14px"
+				}
+			}
+		}
+	}
+}

blockbase/functions.php

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+<?php
+
+if ( ! function_exists( 'blockbase_support' ) ) :
+	function blockbase_support()  {
+
+		// 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( array( 
+			'style.css',
+			'assets/alignments-editor.css'
+		) );
+    }
+    add_action( 'after_setup_theme', 'blockbase_support' );
+endif;
+
+/**
+ * Enqueue scripts and styles.
+ */
+function blockbase_scripts() {
+	wp_enqueue_style( 'blockbase-styles', get_stylesheet_uri() );
+	wp_enqueue_style( 'blockbase-block-styles', get_template_directory_uri() . '/assets/alignments-frontend.css' );
+}
+add_action( 'wp_enqueue_scripts', 'blockbase_scripts' );

blockbase/style.css

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+/*
+Theme Name: Blockbase
+Theme URI: https://github.com/kjellr/blockbase
+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: blockbase
+
+Blockbase is distributed under the terms of the GNU GPL.
+*/