Documentation: fix inconsistent dark mode

[airwoodix]

qiskit-sphinx-theme v2.0 introduces a dark mode for the documentation.

Issues found after only upgrading the dependencies (#193):

• certain page switches (e.g. to the user guide page) do not preserve the light/dark mode setting.
• the jupyter-executed code blocks have wrong color settings in dark mode:
  

compared to regular code blocks, e.g. from docstrings:

[airwoodix]

Will be fixed by Qiskit/qiskit_sphinx_theme#625.