Forms: Improve compatibility with dark themes and outline styles

### Impacted plugin

Jetpack

### Quick summary

Currently when you use a dark theme (such as twenty twenty five with the dark colour pattern) you end up with a buggy design of the Outline style.

See

### Steps to reproduce

1. Activate the twenty twenty five theme. Select the "Evening" colour style.
2. Add a form to the page and select the Outline option.

Notice that there is a white border under the label text.
Notice that the placeholder text is hardly visible.

### Site owner impact

Fewer than 20% of the total website/platform users

### Severity

Moderate

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

_No response_

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

You could apply some custom css to fix this issue.

### Platform (Simple and/or Atomic)

Simple, Self-hosted, Atomic