Customizing the CSS for the post/page editor?

As you can see from the cropped screenshot below, where I have an HD widescreen monitor, I have a very narrow editing box to create or edit posts or web pages.

I have found that the title block’s CSS rule is

.wp-block {
    max-width: 610px;
}

and the main content block’s CSS rule is

.edit-post-text-editor {
    max-width: 610px;
    margin-left: auto;
    margin-right: auto;
}

I would like to adjust the max-width: properties within these rules, so I tried using the ‘Additional CSS’ section of the ‘Appearance Customizer’ but that didn’t work.

How do I customize the post/page editor screen?