Update markdown.py

[pop2pop3]

Add pygments theme reference for code_theme in Markdown

Type of changes

• [ ] Bug fix
• [ ] New feature
• [✓ ] Documentation / docstrings
• [ ] Tests
• [ ] Other

Checklist

• [ ] I've run the latest black with default args on new code.
• [ ] I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• [ ] I've added tests for new code.
• [ ] I accept that @willmcgugan may be pedantic in the code review.

Description

Please describe your changes here. If this fixes a bug, please link to the issue, if possible.