Hiya,
Currently, JupyterLab has 'Settings' menu, which opens 'Settings' tab with 3 categories: CodeMirror, Keyboard Shortcuts, and Text Editor.
CodeMirror settings have entries for theme (JSON-like) and keyMap and theme, while Text Editor has CodeMirror-like entries for autoClosingBrackets, lineNumbers, lineWrap, matchBrackets.
It is a bit confusing, but I assume that CodeMirror is a base for both editor and notebook cells' code.
Also, I see there's in the GitHub repo an extension for JL dark theme, but I don't see a way to change the theme in JL itself.
Cheers,
Milos.