[theme] Implement more minimal styling for sphinx-proof

Thanks @xuanguang-li for raising https://github.com/QuantEcon/lecture-python.myst/pull/533

As part of that review process I think we need to activate minimal styling for `sphinx-proof`

<img width="829" height="688" alt="Image" src="https://github.com/user-attachments/assets/41121a85-e89b-463f-8c23-831b6c5cfebc" />

These assumptions are a wall of colour 😄 