Inconsistent background colors in code sections

[hkosova]

Q&A (please complete the following information)

• OS: Windows 10
• Browser: Chrome latest
• Method of installation: https://petstore.swagger.io
• Swagger-UI version: 3.30.0
• Swagger/OpenAPI version: any

Content & configuration

Example Swagger/OpenAPI definition: see https://petstore.swagger.io

Describe the bug you're encountering

This is a follow up to PR #6236.

Code sections with and without syntax highlighting use different background color, which does not look good. The colors should be consistent.

To reproduce...

Try out any operation in https://petstore.swagger.io.

Expected behavior

The "Example", "Response body", "Response headers", "Curl", and "Request URL" sections should have the same background color.

Screenshots

Additional context or thoughts

Consider rendering the "Request URL" and "Response headers" sections using Highlight.js as well, but with language = plain text (plaintext, txt, or text).