Improve theme integration

[flying-sheep]

Fixes #618

[tasansal]

Hi, I tried this with our docs that use ipywidgets progress bars via tqdm. The text around the progress bar still doesn't honor the color preference from the furo theme when rendered with myst-nb. I just modified the CSS with the one from this PR. Am I missing anything, or is there an issue?

[flying-sheep]

This PR is just for preventing MyST-nb from messing up a theme’s table CSS. I re-titled #618 to be more clear.

For ipywidgets to work properly, the theme needs to integrate with them by setting --jp-widgets-color and so on or by hacking it with a light background.

E.g. sphinx-book-theme currently does

html[data-theme="dark"] .bd-content div.cell_output .text_html:not(:has(table.dataframe)),
html[data-theme="dark"] .bd-content div.cell_output .widget-subarea,
html[data-theme="dark"] .bd-content div.cell_output img {
  background-color: var(--some-light-color);
}

It would have to remove the second selector (.widget-subarea) and define all the --jp-widgets-* variables to really support ipywidgets.

But this PR is a very necessary pre-condition for anything to be able to work. MyST just injecting this CSS is really bad and needs to be fixed!

[flying-sheep]

@bsipocz is there any recorded reason why that CSS exists?

If yes, we need to fix this in another way.

If not, we should really remove this without replacement, just setting styles for div.cell_output table is very bad manners for an extension.