TEST: quantecon-book-theme improvements around collapse- styling

[mmcky]

Our theme somewhere along the way change styles for collapse-## to a scroll bar, when it should be an expand/collapse behaviour.

We have implemented a fix in https://github.com/QuantEcon/quantecon-book-theme/pull/300

I would like to test this by opening a PR and installing this theme from the branch in environment.yml

We can use:

git+https://github.com/QuantEcon/quantecon-book-theme@copilot/fix-299

in the environment.yml for the ci.yml workflow to build a preview.