Feature Request: allow changing input background color for Subscribe block

### Impacted plugin

Jetpack

### What

Allow changing background colour for subscribe block

### How

When Subscribe block is placed against colourful background, the input is transparent:

I would expect input background follow theme's color scheme where possible and user be able to adjust background colour to their liking.

There are also differences across themes on how this behaves, here's Twenty Twenty Two:

**Editor**

Note opaque background.

**Site**

Note solid background.

The Subscribe block is the entry point; start by locating its editor and front-end styling and compare the behavior shown for the Twenty Twenty-Two theme. Define what background-color control should be available and verify the input renders consistently in both editor and site views.

Written by the indexing model from the issue text.