What theme? What button?
Can you verify that the behavior is the same if you go to [yoursitename]/#/portal/ ? [If that isn’t what you’re already doing.] If it’s broken there, then it’s definitely a configuration problem, not a theme problem. If it works in portal, then you’re looking at a theme problem.
send-magic-link is the thing that sends an email to the user so that they can confirm their email address, which makes them a member of your site (and optionally signs them up for your email newsletter).
By default, your Ghost install isn’t configured to send email. You’re going to need to configure at least transactional email, which goes out by SMTP.
Useful links:
Setting up with Mailgun for transactional email is pretty easy (but search the forum for the stupidly weird process for getting onto the Flex plan), or for super fast setup, I’m a big fan of just hooking into email as a temporary fix, although you’d have to translate it to Docker: