Use variable for highlighting code

[shati-patel]

Code highlighting looked funny in light mode 🙈

I've used a variable instead, so it currently looks like this in dark/light mode:

or

My choice of --vscode-editor-findMatchHighlightBackground was fairly random, but it looks plausible 😅

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[charisk]

Thanks, that looks much better! 😄