Force light mode for notebook outputs

[gabalafou]

If you compare the following screenshots, you will find that the some of the Xarray output is invisible in dark mode.

This PR makes a stronger attempt at forcing notebook outputs into light mode by resetting all of the color variables on the notebook output element to light mode.

Before, screenshots

Xarray light mode	Xarray dark mode

[gabalafou]

Switching to draft mode while I work out the merge conflicts

[Carreau]

Should there be an upstream issue in xarray as well ? I'm going to guess this also affect Lab users using dark theme, doesn't it ?

[gabalafou]

No, it doesn't affect JupyterLab users using dark theme.

I think this is because JupyterLab's dark mode CSS is more robust for various widgets than our CSS.

See:

• Implement @smeragoel's table designs #1757 (comment)
• Inproper styling of dataframes for myst-nb + pydata theme when contained in ipywidget (2) #1740 (comment)

[gabalafou]

Should there be an upstream issue in xarray

That said, there are some Xarray CSS rules that are not friendly in dark mode. Here's a screenshot from JupyterLab:

The color of the text "10 20" and the file icon is way too dark to contrast well with a dark background.