<?php
function my_enqueue_block_editor_assets() {
    wp_enqueue_style(
        'my-block-editor-styles',
        get_theme_file_uri( 'block-editor-style.css' ),
        array(),
        filemtime( get_theme_file_path( 'block-editor-style.css' ) ),
        'all'
    );
}
add_action( 'enqueue_block_editor_assets', 'my_enqueue_block_editor_assets' );