Improve output cells in dark mode

[jaladh-singhal]

Fixes #678 - I implemented a mix of both approaches mentioned there (because I couldn't resist noticing more unpolished dark-mode coloring as I delved into it 🧐)

• Updated .cell_output styles to remove borders from plain or stream outputs that contain highlighted blocks (I realized we want to keep borders for traceback and stderr so left them untouched)
• Introduced neutral and danger color palettes (based on Material Design palette generator), replacing hardcoded colors with palette variables for logical and consistent use of colors.
  • Added dark theme overrides for uniform look.

[jaladh-singhal]

For testing: Compare https://myst-nb--679.org.readthedocs.build/en/679/authoring/jupyter-notebooks.html#code-cells-and-outputs with https://myst-nb.readthedocs.io/en/latest/authoring/jupyter-notebooks.html#code-cells-and-outputs in dark mode.

Note how dark mode red border for traceback cell output is darker red shade as it should be in dark theme (unlike a light red shade in light theme)

Also regression test light mode, since I tweaked colors a lil bit to follow the palette.

[bsipocz]

Code looks reasonable to me, and the diff is clearly an improvement so I'm going ahead and merge this now. Thanks @jaladh-singhal!