fix: pop-up in the translation stats viz has dark text over dark background

The pop-up that appears with details when you hover the mouse in the [translation stats viz](https://www.pyopensci.org/python-package-guide/TRANSLATING.html) has black text over a dark grey background.

<img width="837" height="341" alt="Image" src="https://github.com/user-attachments/assets/ce3d51f7-0de1-4da5-abea-c4dfd4c00f6f" />