The pulldown button has this html for the 2.0 documentation

<button id="pst-version-switcher-button-2" type="button" \
   class="version-switcher__button btn btn-sm navbar-btn dropdown-toggle" 
   data-bs-toggle="dropdown" aria-haspopup="listbox" \
   aria-controls="pst-version-switcher-list-2" \
   aria-label="Version switcher list" \
   data-active-version-name="2.0" \
   data-active-version="2.0"> \
2.0 \
</button>

We have this CSS styling in numpy.css

.version-switcher__button[data-active-version-name*="devdocs"] {
  background-color: var(--pst-color-warning);
  border-color: var(--pst-color-warning);
  opacity: 0.9;
}

.version-switcher__button:not([data-active-version-name*="stable"])\
:not([data-active-version-name*="dev"])\
:not([data-active-version-name*="pull"]) {
  background-color: var(--pst-color-danger);
  border-color: var(--pst-color-danger);
  opacity: 0.9;
}

The theme documentation mentions the top CSS stanza, but not the second one. I don't know why #26125 enabled that stanza. It was applied to the 2.0 documentation but not activated due to the logic, but now that the 2.0 documentation is no longer "stable" the styling is activated. I submitted numpy/doc#27 to comment it out (waiting for review), but still do not understand what the motivation for making a version red. @tupui I think this was added in #21451?

Another point, the devdoc button has this html:

<button id="pst-version-switcher-button-2" type="button" \
   class="version-switcher__button btn btn-sm dropdown-toggle" \
   data-bs-toggle="dropdown" aria-haspopup="listbox" \
   aria-controls="pst-version-switcher-list-2" \
   aria-label="Version switcher list" \
   data-active-version-name="dev" \
   data-active-version="devdocs">
dev
</button>

So the CSS attribute does not match the first selector. I think that is due to the versions.json in numpy/doc which somehow gets converted to the data-active attributes, so the selector should be data-active-version-name*="dev":

[
    {
        "name": "dev",
        "version": "devdocs",
        "url": "https://numpy.org/devdocs/"
    },
    {
        "name": "2.1 (stable)",
        "version": "2.1",
        "url": "https://numpy.org/doc/stable/",
        "preferred": true
    },
    {
        "name": "2.0",
        "version": "2.0",
        "url": "https://numpy.org/doc/2.0/"
    },
...

Rather than fix the css selectors, another option would be to delete them and not attempt to colrize the button. We do have the header This is documentation for an unstable development version., so maybe that is sufficient?

do not understand what the motivation for making a version red. @tupui I think this was added in #21451

At the time there was no styling around the button and we wanted to give a visual indication to readers. The colours were chosen to be accessible (from a Nature paper IIRC).

a visual indication to readers

What is the special significance of this combinations that justifies coloring the button red for older versions?

not([data-active-version-name*="stable"])\
:not([data-active-version-name*="dev"])\
:not([data-active-version-name*="pull"])

This is debatable of course. It's just to incentivize more people to use latest always know if what they are looking at is the latest version of the doc. Some blogs or tutorials link to a specific version and people could not realize that something is deprecated/legacy.

Just a note that it should also be possible to use the version warning for old versions using https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/announcements.html#version-warning-banners (I'm not sure why it seems that was disabled for the 2.0 docs)

This is an example of how it shows (very similar to the warning for the dev version: https://pydata-sphinx-theme.readthedocs.io/en/v0.14.4/user_guide/announcements.html)

I'm not sure why it seems that was disabled for the 2.0 docs

Enabling/disabling the warning is not part of the documentation build instructions. What must be done to enable the warning?

In the mean time, until we resolve whether we want the 2.0 docs to be red, I merged numpy/doc#27 to disable the red styling.

I see that the option is enabled in our conf.py:

Note: numpy/doc#27 undid the coloring for the 2.0 docs. But the setting is included in the 2.1 and 2.2 docs.

2.1 is already red, and 2.2 will be once the 2.3 docs are up.