Support JupyterLab dark mode in dashboard plots

When using the dask labextension, plots are hard to read in dark mode because some of the colors don't adjust:


Text colors do change (titles, ticks) but others don't. Even just getting the grid lines to be dark instead of light would be 90% of the way to usability.

That said, I highly doubt there's a way for the dashboard to actually know it's being viewed by JupyterLab in dark mode. Since the plots are all iframe'd, we couldn't even do it with CSS variables that Jupyter sets, right? Maybe the extension could inform the scheduler to please change its dashboard theme?

xref https://github.com/dask/distributed/issues/2305, https://github.com/jupyterlab/jupyterlab/issues/3855