Exactly – use developer tools to build your CSS, then add that CSS to a .css file.
Then you can upload it in the Media Library or upload it to your server (in a child theme or a custom folder).
Then you can do this:
add_action( 'enqueue_block_editor_assets', function() {
wp_enqueue_style( 'my-editor-styles', 'URL TO YOUR .CSS FILE' );
} );