Add Custom Template Class to Block Editor Wrapper in the Backend with Marketers Delight theme so that you can customize the editor using CSS. I use this on https://gauravtiwari.org

add_to_functions.php

add_action(
	'enqueue_block_editor_assets',
	function () {
		$theme = wp_get_theme();
		wp_enqueue_style(
		'block-md-custom-css', '/wp-content/themes/Md/custom-editor.css?v=202310'
		);
		wp_enqueue_script(
		'md-wp-editor-class', '/wp-content/themes/Md/backend-js.js?v=202310');
	}
);

backend-js.js

!function(e){var t={};function n(o){if(t[o])return t[o].exports;var r=t[o]={i:o,l:!1,exports:{}};return e[o].call(r.exports,r,r.exports,n),r.l=!0,r.exports}n.m=e,n.c=t,n.d=function(e,t,o){n.o(e,t)||Object.defineProperty(e,t,{enumerable:!0,get:o})},n.r=function(e){"undefined"!=typeof Symbol&&Symbol.toStringTag&&Object.defineProperty(e,Symbol.toStringTag,{value:"Module"}),Object.defineProperty(e,"__esModule",{value:!0})},n.t=function(e,t){if(1&t&&(e=n(e)),8&t)return e;if(4&t&&"object"==typeof e&&e&&e.__esModule)return e;var o=Object.create(null);if(n.r(o),Object.defineProperty(o,"default",{enumerable:!0,value:e}),2&t&&"string"!=typeof e)for(var r in e)n.d(o,r,function(t){return e[t]}.bind(null,r));return o},n.n=function(e){var t=e&&e.__esModule?function(){return e.default}:function(){return e};return n.d(t,"a",t),t},n.o=function(e,t){return Object.prototype.hasOwnProperty.call(e,t)},n.p="",n(n.s=5)}([function(e,t){e.exports=function(e,t){if(!(e instanceof t))throw new TypeError("Cannot call a class as a function")}},function(e,t){function n(e,t){for(var n=0;n<t.length;n++){var o=t[n];o.enumerable=o.enumerable||!1,o.configurable=!0,"value"in o&&(o.writable=!0),Object.defineProperty(e,o.key,o)}}e.exports=function(e,t,o){return t&&n(e.prototype,t),o&&n(e,o),e}},function(e,t,n){var o=n(6),r=n(7);e.exports=function(e,t){return!t||"object"!==o(t)&&"function"!=typeof t?r(e):t}},function(e,t){function n(t){return e.exports=n=Object.setPrototypeOf?Object.getPrototypeOf:function(e){return e.__proto__||Object.getPrototypeOf(e)},n(t)}e.exports=n},function(e,t,n){var o=n(8);e.exports=function(e,t){if("function"!=typeof t&&null!==t)throw new TypeError("Super expression must either be null or a function");e.prototype=Object.create(t&&t.prototype,{constructor:{value:e,writable:!0,configurable:!0}}),t&&o(e,t)}},function(e,t,n){"use strict";n.r(t);var o=n(0),r=n.n(o),u=n(1),i=n.n(u),c=n(2),p=n.n(c),l=n(3),f=n.n(l),s=n(4),a=n.n(s),y=wp.element.Component,b=wp.compose.compose,d=wp.data.withSelect,m=wp.plugins.registerPlugin,v=function(e){var t=e.replace(/\./g,"-").replace(/[!\"#$%&'\(\)\*\+,\/:;<=>\?\@\[\\\]\^`\{\|\}~]/g,"");return"page-template-".concat(t)},g=function(e){function t(){return r()(this,t),p()(this,f()(t).apply(this,arguments))}return a()(t,e),i()(t,[{key:"componentDidMount",value:function(){var e=document.querySelector(".editor-styles-wrapper");this.props.selectedTemplate&&e.classList.add(v(this.props.selectedTemplate))}},{key:"componentDidUpdate",value:function(e){var t=document.querySelector(".editor-styles-wrapper");t.classList.remove(v(e.selectedTemplate)),this.props.selectedTemplate&&t.classList.add(v(this.props.selectedTemplate))}},{key:"render",value:function(){return null}}]),t}(y);m("bigbox-editor-page-template-class",{render:b(d(function(e){return{selectedTemplate:(0,e("core/editor").getEditedPostAttribute)("template")}}))(g)})},function(e,t){function n(e){return(n="function"==typeof Symbol&&"symbol"==typeof Symbol.iterator?function(e){return typeof e}:function(e){return e&&"function"==typeof Symbol&&e.constructor===Symbol&&e!==Symbol.prototype?"symbol":typeof e})(e)}function o(t){return"function"==typeof Symbol&&"symbol"===n(Symbol.iterator)?e.exports=o=function(e){return n(e)}:e.exports=o=function(e){return e&&"function"==typeof Symbol&&e.constructor===Symbol&&e!==Symbol.prototype?"symbol":n(e)},o(t)}e.exports=o},function(e,t){e.exports=function(e){if(void 0===e)throw new ReferenceError("this hasn't been initialised - super() hasn't been called");return e}},function(e,t){function n(t,o){return e.exports=n=Object.setPrototypeOf||function(e,t){return e.__proto__=t,e},n(t,o)}e.exports=n}]);

custom-editor.css

.editor-styles-wrapper.page-template-premium-builder-php, .editor-styles-wrapper.page-template-builder-php {
    background: #f3f4f6;
    width: 100%;
}

.editor-styles-wrapper.page-template-premium-builder-php .wp-block, .editor-styles-wrapper.page-template-builder-php .wp-block{
    max-width:1200px;
}
/* Use your own CSS */