fix: improve dark mode neutral background contrast

[jzempel]

Description

Applies requested design improvements to:

• Progress
• Skeleton
• all disabled form input backgrounds (Button, Combobox, Input, Radio, etc)

Checklist

• 👌 design updates will be Garden Designer approved (add the designer as a reviewer)
• 🌐 demo is up-to-date (npm start)
• ⬅️ renders as expected with reversed (RTL) direction
• ⚫ renders as expected in dark mode
• 🤘 renders as expected with Bedrock CSS (?bedrock)
• 💂‍♂️ includes new unit tests. Maintain existing coverage (always >= 96%)
• ♿ tested for WCAG 2.1 AA accessibility compliance
• 📝 tested in Chrome, Firefox, Safari, and Edge

[coveralls]

coverage: 95.523%. remained the same
when pulling 2739a7b on jzempel/color-tweaks
into 597e978 on main.