Fix color contrast issue in SwiftUI code snippets

[laylahoogeveen]

What does this PR do?

Change the colour for interpolated values in code blocks, so the text is visible in light mode. (https://github.com/appt-org/appt-website/issues/62)

How do you test this PR?

Test link: http://localhost:3000/en/docs/swiftui/samples/accessibility-value

I haven't found any other code samples with an interpolation value, so I'm afraid we can only test the updated css with this doc. The text is rendered black on a white surface (light theme) and white on a black surface (dark theme), so I'm pretty sure this won't cause any problems whatsoever.

Test steps:

• View in light mode
• View in dark mode
• Is everything readable?

Checklist

• [ ] Works on Safari, Chrome, Firefox & Edge
• [ ] Works on different screen sizes (responsive)
• [ ] Scores 100 in Lighthouse/PageSpeed Insights on SEO, accessibility and best practices, and >= 90 on performance
• [ ] Does not contain tracking cookies
• [ ] Are migrations added?
• [ ] Is the README.md updated?

Accessibility checklist

• [ ] Scalable text
• [ ] Dark and light mode
• [ ] Contrast
• [ ] Focus management
• [ ] Aria attributes