I noticed a problem with the furo theme example: The background color for stderr output isn't compatible with the dark scheme:

but using a transparency with slightly increased saturation should remedy that in a consistent way for light or dark schemes:
/* nbsphinx-code-cells.css | https://nbsphinx.readthedocs.io/en/furo-theme/_static/nbsphinx-code-cells.css */
div.nboutput.container div.output_area.stderr {
/* background: #fdd; */
background: #e8808075;
}

There are other styles that use opaque hard-coded colors which don't blend well with light and dark schemes. I'll update this thread with other related findings...
I noticed a problem with the furo theme example: The background color for stderr output isn't compatible with the dark scheme:

but using a transparency with slightly increased saturation should remedy that in a consistent way for light or dark schemes:
There are other styles that use opaque hard-coded colors which don't blend well with light and dark schemes. I'll update this thread with other related findings...