Open in new window / tab icon on link doesn't turn purple when visited in Safari

[GCHQ-Developer-847]

Summary of the bug

The 'open in new window / tab' icon which can be displayed next to a link does not purple when the link is visible in Safari.

🪜 How to reproduce

1. On Safari, find an external link e.g. on the website (at the bottom of the page).
2. Click on the link (to make it become a visited link).
3. Go back to the link and see that the text has turned purple but the icon is still blue.

📸 Screenshots or code

🖥 📱 Device

• Browser: Safari

🧐 Expected behaviour

The icon should turn purple, matching the text.