Add new focus theme variable for improved color theme support

[pastelcyborg]

Done

• Added new $colors--theme--focus color variable
• Updated all existing uses of $color-focus to $colors--theme--focus

Fixes WD-12630

QA

• Open switch, button base, button default, form tick variants, and accordion demos
• Test that focus outlines look as expected across all themes
• Verify any other components that have focus state outlines

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 release (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.

[webteam-app]

Demo

Jenkins

demos.haus

[bartaz]

LGTM