Fix the colour scheme for code examples

[NevilleS]

This fixes the missing code here:

The "variable" colour was blending into the background. Fixed version:

As a bonus, I also removed the openapi.json from being committed to source, as this is supposed to be a build artifact and is ignored in .gitignore.

Code Changes

• Update fides.css

Steps to Confirm

• Tested with make docs-serve

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation Updated