WordPress Gutengerg starter editor stylesheet. See https://sebastien-meric.com/wordpress-gutenberg-feuille-de-style-editeur/ (in french).

editor-style.css

/**
 * Gutenberg backoffice common content stylesheet
 * 
 * Do note that every CSS declarations will be prepended with .editor-styles-wrapper class.
 **/

/** =Bugs
 *************************************/

/* @see https://wordpress.org/support/topic/many-new-bugs-after-update/ */
.block-editor-writing-flow__click-redirect{min-height:auto;height:40vh;}


/** =Base
 *************************************/
 
/**
 * body will be replaced with the .editor-styles-wrapper class.
 */
body {
    width: 100%;
    padding-right: 0;
    padding-left: 0;
}

/* Center title and blocks list wrappers inside Body */
@media (min-width: 600px){
    .editor-post-title,
    .block-editor-block-list__layout {
        /* Max content size */
        max-width: 1170px;
        /* Space around */
        padding-left: 15px;
        padding-right: 15px;
        /* Center the content */
        margin-right: auto;
        margin-left: auto;
    }
}

/* Post title */
.editor-post-title__block {
    background-color: black; /* as an example... */
    width: 100%;
    height: 100%;
    margin: 0;
    padding: 60px 30px; /* as an example... */
    }
    @media (min-width: 600px){
        .editor-post-title__block > div {
            margin-left: 0;
            margin-right: 0;
            }
        }
        .editor-post-title__input {
            margin-top: 0;
            margin-bottom: 0;
            padding:0;
            opacity: 1 !important;
        }

/* Remove blocks default max-width */
.wp-block {
    max-width: none;
}

/**
 * Add all theme content specific styles here :
 */

/** =Typography
 *************************************/
 
/* Post title */
.editor-post-title__block {
    }
    .editor-post-title__input {
        font-family: 'Poppins', sans-serif;
        text-align: center;
        color: white;
        font-size: 25px;
        text-transform: none;
        letter-spacing: 1px;
        font-weight: 500;
        line-height: 1.2;
    }
    .editor-post-title__input:focus {
        color: white;
    }
    .editor-post-title__input::-webkit-input-placeholder{
        color: rgba(255,255,255,.8);
    }
    .editor-post-title__input:-moz-placeholder{
        color: rgba(255,255,255,.8);
    }
    .editor-post-title__input::-moz-placeholder{
        color: rgba(255,255,255,.8);
    }
    .editor-post-title__input:-ms-input-placeholder{
        color: rgba(255,255,255,.8);
    }
    .editor-post-title__input::placeholder {
        color: rgba(255,255,255,.8);
}

my_theme_setup_theme.php

<?php
/**
 * Add editor styles to Gutenberg using the editor-style.css stylesheet found
 * at the root of the site active theme directory.
 **/

function my_theme_setup_theme() {
    // Gutenberg stylesheet
    add_theme_support( 'editor-styles' );
    add_editor_style( 'editor-style.css' );
}
add_action( 'after_setup_theme', 'my_theme_setup_theme' );
?>