Rendered code blocks lack backgrounds in JupyterLab 4.3.0+

[dlqqq]

Description

Code blocks lack gray backgrounds in JupyterLab 4.3.0+ due to CSS changes introduced by JupyterLab 4.3.0. This makes messages hrader to read as they blend in with the code blocks.

Reproduce

1. Upgrade JupyterLab to v4.3.0+.
2. Run Jupyter AI off of this branch: [v3-dev] Initial migration to jupyterlab-chat jupyter-ai#1043
3. Make Jupyternaut generate a code block
4. Verify that the error exists

Proposed solution

Port jupyterlab/jupyter-ai#1104 to @jupyter/chat.

[brichet]

Thanks @dlqqq for opening this, and providing a solution.