Icons on cards aren't visible on horizontal + vertical card component pages

[GCHQ-Developer-847]

Summary of the bug

Icons on cards aren't visible on the website in the card code examples.

🪜 How to reproduce

Tell us the steps to reproduce the problem:

1. Go to the horizontal / vertical card component pages.
2. Go to the code examples.
3. See that all the displayed cards are just showing a gap where the icon should be.

📸 Screenshots or code

E.g.

🧐 Expected behaviour

The icons should be visible on the card examples. Need to make sure the icons changing colour when changing between light and dark mode too.