Editor text unreadable in offcanvas (layout builder)

Problem/Motivation

CodeMirror Editor contents are not readable and thereby not usable in the Layout Builder sidebar:

The marked line contains text, but you can't see it.

Text and background have the same color, so the text is completely unreadable and thereby unusable. Similar issue existed in CKeditor: #2952390: Off-canvas styles override CKEditor's reset and theme → and there are some core issues I linked, but as they have a long history, we shouldn't wait for core to fix it.

Steps to reproduce

Use CodeMirror in a layout builder sidebar.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes