Clean language code examples have no border

Summary

Presumably because the clean class is used for other purposes, code examples in the programming examples have no border while others do:

This is clean code

This is javascript code

Screenshot of the above (for reference once the bug is fixed):

Steps to reproduce

The examples above use the markdown:

```clean
This is clean code
```

```js
This is javascript code
```

Example Project

N/A.

What is the current bug behavior?

Code examples in the Clean programming language have no border, while those in other languages do.

What is the expected correct behavior?

Code examples in the Clean programming language have a border as well.

Relevant logs and/or screenshots

See above.

Output of checks

N/A.

Results of GitLab environment info

N/A.

Results of GitLab application Check

N/A.

Possible fixes

🤷