Notebook download outputs have white background in dark mode

[vcatlett]

Describe the bug
In the notebooks rendered in the docs, the output from cells that download data via dysh.util.download.from_url have a white background in dark mode.

How to Reproduce
See this page in dark mode: https://dysh.readthedocs.io/en/cat-docs-devel/examples/frequencyswitch.html#first-we-download-the-example-sdfits-data-if-necessary

[astrofle]

This is the default behavior for widgets. See for example: https://myst-nb.readthedocs.io/en/latest/render/interactive.html