Mailchimp Block Button cannot be centered on old themes. Styling broken on newest themes.

#### Steps to reproduce

1. Add a Mailchimp Block to a page, click on the button, and center it. Publish the page.
2. Activate an older theme (pre-Gutenberg). For example: Prosperity, Colinear, Revelar, Canape, Venture, Veggie, Appetite, Maisha, etc...
3. Refresh the page. The button will not be centered.

4. Activate a newer theme (block-based). For example: Balasana, Maywood, Rivington, Barnsbury
5. Refresh the page. The button will be centered (as it is in the editor):


6. Try an even newer theme. For example, Twenty Twenty, Seedlet, or Spearhead, and they'll come with their own formatting/stylistic issues for that same block:


#### What I expected
The Block to appear on the page styled like it is in the editor.

#### What happened instead
It varies. On our newest themes, the block's styling it all over the place. On older themes, it doesn't match the editor.

#### Browser / OS version
All

#### Screenshot / Video
Screenshots above.

#### Context / Source
Reported this as a theme bug [here](https://github.com/Automattic/themes/issues/2904), but was told it was a block bug, so reposting here.

Reproduce the Mailchimp Block button alignment across the older and newer themes listed in the issue, comparing the published page with the editor. Investigate the block's frontend styling and theme interaction; done means the button matches the editor's alignment and styling across the reported theme groups.

Written by the indexing model from the issue text.