Comment Form: Submit button does not match theme styles

### Impacted plugin

Jetpack, mu wpcom

### Quick summary

When a theme's color styles are set in the Site Editor, the selected color is applied to buttons across the site. On WordPress.com hosted sites, the comment form's Submit button does not render in the displayed color, and instead uses a default blue.

This appears to be specific to WordPress.com, as I wasn't able to reproduce it on a Jurassic Ninja instance. On WordPress.com, the issue did not appear to be theme-specific.

### Steps to reproduce

1. On any WordPress.com hosted site, open the site editor, select **Colors** and choose a palette that makes the buttons appear in a recognizable color. Preferably not blue 😆
2. Create and publish a new post and add a `Buttons` block with at least one button
3. View your new post, note the color of the button in the post content
4. Scroll down and enter some text in the comment field to enable the **Comment** button. Note the button color here as well.

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

I expected the comment form button to match the color declared by the theme/style selection (which I think it used to, if I'm not mistaken)

### What actually happened

The button on the comment form does not match, and renders in one of our default blues. Which, don't get me wrong, is a lovely blue. Truly stunning. Props to the designers on that blue, truly. It's just... not _matchy_ ya know?

### Impact

Most (> 50%)

### Available workarounds?

Yes, difficult to implement

### Platform (Simple and/or Atomic)

Simple, Atomic

### Logs or notes

Reproduced on Twenty Twenty Four and Infield themes.

Run the reproduction steps on a WordPress.com hosted site using the Twenty Twenty Four or Infield theme, comparing the post button with the comment form button. Trace the comment form button's theme-style handling in the Jetpack/WordPress integration. Done means the comment button uses the selected theme color on the affected hosted platforms.

Written by the indexing model from the issue text.