[Visual Refresh] Fix strong border token naming

[mgadewoll]

Summary

We noticed that there seems to have been some typos in the borderStrong SCSS tokens 🫠
This causes the variables to not actually be available for use.

Changes

• renames SCSS borderStrong tokens for Borealis dark
• adds missing !default flag (since we're here 🤷‍♀️)

QA

• verify the tokens are available:

  • checkout this PR
  • use the border tokens, e.g. $euiColorBorderStrongSuccess in our EUI docs, e.g. in packages/eui/src-docs/src/views/home/_home.scss to add a background color somewhere

  // example
  .guideSideNav {
    background-color: $euiColorBorderStrongSuccess !important;
  }

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_8262/
• Storybook: https://eui.elastic.co/pr_8262/storybook
• New docs: https://eui.elastic.co/pr_8262/new-docs

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 5e2b01a

History

• 💔 Build #3308 failed 5e2b01a

cc @mgadewoll

[weronikaolejniczak]

LGTM 👍🏻

I ran the docs website and verified that the new tokens (euiColorBorderStrongPrimary, euiColorBorderStrongAccent etc are available) both in Borealis and Amsterdam.