Is there a way to theme the register page on a network site?

Is there a way to theme the register page (wp-signup.php) for a network site? I could not find any resources on how to do this for a normal site or network site.

I found a guide to sort of theme the login page here but clicking the register link any theming is lost and that is probably because that is the wp-login.php page.

Update:
Looking at the wp-signup.php page it looks like it writes out the page markup inline and then uses the following function for the CSS:

It looks like it is checking if it’s not multisite it redirects to the wp_registration_url().

And it seems to put that code as if it is a post because all of the other components of the site are there.

Compared to the login page that simply has the login form and nothing else.

Update:
It seems like maybe I might be able to add theming support if I can call hasThemePart() and then if true add getThemePart() to add CSS or theme markup. Not sure if the function names are correct but I think I’ve seen them before.