<?php

add_action('enqueue_block_assets', function () {
    // Dequeue default block styles. Must be included in the asset build.
    wp_dequeue_style('wp-core-blocks');
});

<?php

add_action('enqueue_block_assets', function () {
    // Dequeue default block styles. Must be included in the asset build.
    // Must probably be extended to also dequeue additional block styles
    // registered by plugins etc.
    wp_dequeue_style('wp-block-library');
});

Fix dequeuing of default gutenberg block styles
Fix dequeuing of default gutenberg block styles
