feat: add CSS variable `--lime-focus-ring-color`

[adrianschmidt]

Review:

• [ ] Commits are atomic
• [ ] Commits have the correct type for the changes made
• [ ] Commits with breaking changes are marked as such

Browsers tested:

(Check any that applies, it's ok to leave boxes unchecked if testing something didn't seem relevant.)

Windows:

• [ ] Chrome
• [ ] Edge
• [ ] Firefox

Linux:

• [ ] Chrome
• [ ] Firefox

macOS:

• [ ] Chrome
• [ ] Firefox
• [ ] Safari

Mobile:

• [ ] Chrome on Android
• [ ] iOS