Focus not always visible on JupyterLab initial page

[gabalafou]

Description

On the initial page in JupyterLab, some elements that receive focus by pressing the tab key do not have a visible focus indicator, which is a violation of WCAG 2.4.7.

This issue is about fixing those focus indicators for the initial page in JupyterLab.

Value / benefit

Understanding WCAG 2.4.7 explains why.

Implementation details

No response

### TODO
- [ ] https://github.com/Quansight-Labs/jupyter-a11y-mgmt/issues/199
- [x] Submit PR(s) that make the test pass
- [ ] https://github.com/Quansight-Labs/jupyter-a11y-testing/pull/33

Updates

No response

[gabalafou]

jupyterlab/jupyterlab#13415