Border styling for contact form fields not applying in the Quadrat theme.

### Impacted plugin

None / Other

### Quick summary

When setting up borders around contact form fields in the Quadrat theme, the border styles do not appear on the live site. This issue is specific to the Quadrat theme.
Tested both Simple and Atomic and was able to replicate the issue.

User reported this here: 9369670-zd-a8c

### Steps to reproduce

1. Activate the Quadrat theme on a WordPress.com site.
2. Create a new page or edit an existing page (e.g., Contact page).
3. Add a Form block to the page.
4. Customize the form fields by adding a border color.
5. Save and preview the page on the live site.

**Expected result:** The border styles applied to the form fields should be visible on the live site.
**Actual result:** The border styles do not appear on the live site.

**Example 1 (Quadrat Theme Active)**

https://github.com/user-attachments/assets/cc698f22-fa13-4a4f-b28d-57455f56f7c4

**Example 2 (Twenty Twenty-Five Theme Active)**

https://github.com/user-attachments/assets/313f9509-2baa-4d79-b9c1-1b553a4704c9

### Site owner impact

Fewer than 20% of the total website/platform users

### Severity

Minor

### What other impact(s) does this issue have?

No revenue impact

### If a workaround is available, please outline it here.

Instead of applying borders, use the "Field Background" option in the Form block settings to style the form fields.

### Platform (Simple and/or Atomic)

Simple, Atomic

Reproduce the issue with the Quadrat theme and a Form block, following the listed steps and comparing the live result with the expected border styling. Trace how the Form block's border color setting is applied in Quadrat and confirm the fix by verifying that borders appear on the live site without affecting the other tested themes.

Written by the indexing model from the issue text.