Purpose: Subscription Form Block submit button is misaligned with the email input field

### Steps to Reproduce

1. Apply the Purpose theme to the site
2. Add a Subscription Block

### What I expected to happen

The Submit Button should be correctly aligned with the input field, either displaying on the same line, or below it.

### What actually happened

The Submit button displays with a weird alignment.


### Context

Customer report #4234458-zen

### Operating System

macOS

### OS Version

11.15.1

### Browser

Chrome/Chromium

### Browser Version(s)

92

### Is this specific to applied theme? If so, what is the theme name?

Purpose

### Simple/Atomic

Simple, Atomic

### Console and/or error logs

_No response_

### Number of Users Impacted

All users

### Available Workarounds

Provided this CSS, but there might be a better way:

```
@media screen and (min-width: 768px) {
#subscribe-submit button {
margin-top: auto;
}}
```

### Reproducibility

Consistent

### Other information

_No response_

Reproduce the issue by applying the Purpose theme and adding a Subscription Block, checking the layout at the desktop breakpoint described by the workaround. The work is done when the submit button aligns with the email input, either on the same line or clearly below it, without the reported misalignment.

Written by the indexing model from the issue text.