Refactor themes

[trexfeathers]

🚀 Pull Request

Description

See project item

To do

• Tests
• Did we want a docs page outside the API docs themselves?

[github-actions]

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[tkoyama010]

This is still important.

[github-actions]

In order to maintain a backlog of relevant PRs, we automatically label them as stale after 180 days of inactivity.

If this PR is still important to you, then please comment on this PR and the stale label will be removed.

Otherwise this PR will be automatically closed in 28 days time.

[trexfeathers]

For the record I consider this blocked by pyvista/pyvista#5549, at least if we want to do things 'properly'.

If we were willing to just write our own version of what is happening in PyVista, we could proceed now. Of course that would break the 'thin wrapper' philosophy, and leave us vulnerable to divergence.