Mailchimp: Adjust Input field appearlance

### Impacted plugin

Jetpack

### Quick summary

When you add a Mailchimp block, this is how it appears with TT3.


This is far from ideal. Especially, with the lack of custom options. Compare to the default state of the search block.


Can we make it at least the same as the search block?

Also the 2px border seems random. Can we make the border editable in Global Styles? Otherwise, themes needs to style it with css, which is not ideal.

cc @jeffikus

### Steps to reproduce

1. Add the Mailchimp block to the content in the editor.

### A clear and concise description of what you expected to happen.

_No response_

### What actually happened

_No response_

### Impact

All

### Available workarounds?

Yes, difficult to implement

### Platform (Simple and/or Atomic)

_No response_

### Logs or notes

_No response_

Start by locating the Jetpack Mailchimp block and compare its input appearance with the default Search block in TT3. Check how the block's border and Global Styles support are defined; done means the input matches the Search block's baseline presentation and its border can be edited through Global Styles without theme-specific CSS.

Written by the indexing model from the issue text.