Fix code blocks in dark mode (#733)

### Prerequisites

Put an `x` into the box(es) that apply:

- [x] This pull request fixes a bug.
- [ ] This pull request adds a feature.
- [ ] This pull request introduces breaking change.

### Description

In dark mode, the `code` tag inside `pre` in source code blocks has a
different background color, resulting in two different colors like the
following example:


![codeblocks](https://user-images.githubusercontent.com/26819407/202913094-757949fa-feb6-43a3-ae01-2a3a9ed6b5ee.png)

The bug seems to have been introduced by commit
476429d42ac1971595ccd44a8232685a5ee86d8a.

I have fixed it by adding `inherit` colors to `code` tags that are
inside `.highlight` classes.

### Issues Resolved

No issue has been filed.

### Checklist

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#### General

- [x] Describe what changes are being made
- [x] Explain why and how the changes were necessary and implemented
respectively
- [ ] Reference issue with `#<ISSUE_NO>` if applicable

#### Resources

- [x] If you have changed any SCSS code, run `make release` to
regenerate all CSS files

#### Contributors

- [ ] Add yourself to `CONTRIBUTORS.md` if you aren't on it already