Documentation - Zero to thunder rendered as white font on white background

## 🐛 Bug

Part of the documentation (outputs of computation cells) is invisible.

### To Reproduce

Steps to reproduce the behavior:

1. Open [Zero to thunder tutorial](https://lightning-thunder.readthedocs.io/en/latest/notebooks/zero_to_thunder.html)
2. Try to read outputs of cells.

### Expected behavior
Outputs are visible.

### Workaround
Refreshing a page a few times sometimes fixes the issue.

### Environment

- OS (e.g., Linux): MacOS
- Browser: Chrome, Opera, Safari (I was able to reproduce the issue in these 3 browsers)

### Additional context

Screenshot: on the left the text is invisible, on the right it is visible.


cc @borda @apaz-cli