CM6: Fix background color

[vidartf]

In lab 4, the theme for a code editor is only scoped on the notebook and console classes (in a bit of a bleedthrough in the lab codebase: the specific extensions "notebook" and "console" are special cased in the generic "codemirror" package):

https://github.com/jupyterlab/jupyterlab/blob/1d989bad87e89307a336d2f671c9cc98ec37f5e9/packages/codemirror/src/theme.ts#L27C1-L29

We should either see if we can add to the global theme such that our diff/merge views also get this styling, or create a specific selector outside of the theme system.

[HaudinFlorence]

We should either see if we can add to the global theme such that our diff/merge views also get this styling, or create a specific selector outside of the theme system.

@vidartf I can try to work on it. Is there a special reason to have a grey background and not something closer to notebook cells ? What is best to choose between the 2 options mentioned above.

cc @fcollonval @krassowski

[vidartf]

grey background and not something closer to notebook cells ?

To clarify: Notebook cells do have a gray background. Any usage of codemirror outside of notebook and console on the other hand now (with lab 4) has a default background of white. We should try to mimic the look in notebooks as much as possible, so we should also use the same theming.

[vidartf]

What is best to choose between the 2 options mentioned above.

The first option is probably more robust / future-proof, but the second option will be simpler to implement (the way CM styling works now).

[HaudinFlorence]

@vidartf Thanks for your input and for the precisions. I opened this PR #714 to try to fix the issue.

[HaudinFlorence]

I think that this issue can be closed.

[vidartf]

Thanks