Level 3 and level 2 headings are hard to distinguish

[HuangJunye]

Description

Level 3 and level 2 headings are hard to distinguish because they use the same font with similar size. Example: https://qiskit.org/documentation/apidoc/circuit.html#quantum-circuit-api

Suggested solution

Modify the size and / or use bold / italitc and / or colors to distinguish level 2 and level 3 headings. This may also apply to other levels of headings to make each easier to distinguish.

[Eric-Arellano]

Fixed by new Furo theme (#425)