Try to read Jupyter CSS properties even if body attribute is not set.

[fcollonval]

In https://github.com/jupyterlab/jupyterlab/pull/15021, styles are not computed for the examples not using the theme manager.

[github-actions[bot]]

:point_left: Launch a Binder on branch jupyterlab-contrib/jupyter-ui-toolkit/fix/load-jp-css-anyway