Raise the contrast of form fields, list numbers, the calendar and outline hover

[MaggieCabrera]

Description

Part of #44. Five things in the theme's styles fell below the WCAG AA contrast minimums in most color variations. This raises them:

• Form field borders (comment form, search, Jetpack form) go from theme-4 to theme-2, and to theme-6 on hover. They were 1.5–2:1, and need 3:1. They're now 5.85:1 or more.
• The search placeholder goes from 0.6 to 0.9 opacity. It was 2.7–3.8:1, and needs 4.5:1. It's now 4.99:1 or more, and still lighter than typed text.
• Ordered list numbers use the text color instead of theme-4, which was 1.5–2:1. Bullets stay faint, since they don't carry information.
• The calendar's weekday row sits on theme-5 instead of theme-4, like table headers. It was 3.87:1 in After Hours.
• The outline button's hover text goes to theme-6, like links. On a theme-5 surface in After Hours it was 4.32:1, and it's now 9.27:1.

The black-on-black "Large title" pattern (#46) is a bug in the core pattern, so it's fixed upstream in WordPress/wordpress-develop#13793 (Trac #66201) and this PR doesn't touch it.

Screenshots

	Before	After
Comment form, Default
Comment form, After Hours
Search results, Default
Search results, Sunrise
CTA Search, Default
CTA Search, Golden Hour
Ordered list, Default
Ordered list, Evening
Calendar, After Hours
Calendar, Default
CTA Membership "View plans" on hover, After Hours
Core "Large title" pattern with WordPress/wordpress-develop#13793, Default (not part of this PR)

Testing Instructions

1. Import the accessibility test content.
2. Open "Template: Comments", search for nothing (/?s=), and open "WP 6.1 Text category blocks" and "WP 6.1 Widgets block category". Field borders and the placeholder are easy to see, list numbers match the text, and the calendar's weekday row reads well.
3. Insert the CTA Search and CTA Membership patterns in a page.
4. Switch to After Hours in Styles and hover "View plans" in CTA Membership. The text stays readable.
5. Repeat with Sunrise and Evening, and check the same pages in the Site Editor.

I checked the front end and the Site Editor in all eight color variations, measuring every text, border and button state on those pages and on a page with all of Ipsum's and core's patterns.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Auditing contrast across the color variations, implementing the change and taking the screenshots.

🤖 Generated with Claude Code

[MaggieCabrera]

@henriqueiamarino if you could have a look to check on these changes, since we are making color changes?

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Co-authored-by: MaggieCabrera <onemaggie@git.wordpress.org>
Co-authored-by: henriqueiamarino <iamarinoh@git.wordpress.org>

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[henriqueiamarino]

Hey @MaggieCabrera, the changes hurt the default style a bit, but I believe they benefit the others a lot. With that, we can proceed with the changes.