Fix code highlight bg override variable name

[mattarau]

What does this PR cover?

It fixes a wrong variable name for override_code_highlight_bg in the index.html file.

How can this be tested?

1. Make sure you have a light code_highlight_theme defined (the default github will work).
2. Change override_code_highlight_bg to a dark color (i.e. #090B0C).
3. Make sure you have a component with code sample defined.
4. Open the library pattern on the browser.
5. Click on "Show Code Sample" from any component to see that the defined background override is now applied.

Screenshots / Screencast

After following the above steps (but using vs as code_highlight_theme):

Observation

I haven‘t created an issue before sending this PR (because of the simplicity of the change). To reproduce the error, you can follow the steps above and check that the dark background is not applied:

---

Reviewers

Review 1

• [ ] :+1:

Review 2 (optional)

• [ ] :+1:

By adding a +1 you are confirming you have...

• Witnessed the work behaving as expected (this could be on the author's machine or screencast).
• Checked for coding anti-patterns.

---

[Andy-set-studio]

Hey @mdentinho,

As you can see, there's a conflict here. It's to do with the highlight.js reference. On your fork it's referencing 9.3.0, but on our master it's referencing 9.12.0.

If you could add that change to this PR, I'll review it again.

Thanks very much for contributing!