Add hook on theme change to allow extensions to adapt their themes.

[Carreau]

Mostly thinking of jupyter-lite-sphinx which wants to sync the JupyterLab theme with pydata one.

I'm not sure this is the right approach, but at least that should get the discussion started.

[Carreau]

This should allow something like:

    if (window.pst_theme_change_hooks == undefined) {
        window.pst_theme_change_hooks = [];
    }
    window.pst_theme_change_hooks.push(function(evt){console.log('switched to theme', evt)});

Or should we have a register_hook function that ensure that the hook re triggered at least once when they are registered after the mode has already be set ?

[gabalafou]

Related:

• React to pydata-sphinx-theme theme change jupyterlite/jupyterlite-sphinx#69

[gabalafou]

If I understand this approach correctly, it requires code in jupyter-lite-sphinx or jupyterlite to register hooks with our code.

There are two reasons why I am opposed to this approach:

1. I believe it is not in the PyData Theme's best interest to go down the path of having a JavaScript API (in some shape or form) to maintain. As this is a theme, I think our attention would be better focussed on working well with Sphinx, making sure that our HTML and CSS produces aesthetically pleasing and accessible web pages, as well as some other things that to me seem more properly related to shipping a good theme. Maintaining JavaScript that is intended to be called by others is adding maintenance costs that could distract from the core concerns of the theme.
2. As JupyterLite is much, much more JavaScript heavy (and savvy) than PyData Theme, I think it makes sense for us to call them on dark/light mode change, not for them to call us.

As we discussed in yesterday's czi-scientific-python sync, I'm closing this issue in favor of exploring other alternatives that are being discussed in jupyterlite/jupyterlite-sphinx#69.

[gabalafou]

I should add that your PR did have the intended effect of generating discussion 😉