Dequeue the WordPress block editor stylesheet if the Classic Editor plugin is activated and configured

dequeue-wordpress-block-library-styles.php

add_action( 'wp_enqueue_scripts', 'simon_dequeue_block_editor_styles' );

function simon_dequeue_block_editor_styles() {

	if ( 'classic' === get_option( 'classic-editor-replace' ) ) {
		wp_dequeue_style( 'wp-block-library' );
	}

}