Add new theme support for base forms

[britneywwc]

Done

• Add CSS variables theme support for base forms
• Fixed a minor typo
• Note: base forms now use border-high-contrast for the light & paper theme

Fixes WD-7596

QA

• Open demo
• Go to /docs/example
• Make sure the base forms are consistent with their themes

Check if PR is ready for release

If this PR contains Vanilla SCSS code changes, it should contain the following changes to make sure it's ready for the release:

• PR should have one of the following labels to automatically categorise it in release notes:
  • Feature 🎁, Breaking Change 💣, Bug 🐛, Documentation 📝, Maintenance 🔨.
• Vanilla version in package.json should be updated relative to the most recent release, following semver convention:
  • if CSS class names are not changed it can be bugfix relesase (x.x.X)
  • if CSS class names are changed/added/removed it should be minor version (x.X.0)
  • see the wiki for more details
• Any changes to component class names (new patterns, variants, removed or added features) should be listed on the what's new page.

Screenshots

[if relevant, include a screenshot or screen capture]

[webteam-app]

Demo starting at https://vanilla-framework-4974.demos.haus

[bartaz]

Awesome, thanks!