REGR: Subclassing `Styler` and addressing `from_custom_template`
- [x] closes #42053

More info there - note that this makes the broken functionality re-available but is not backwards compatible due to inherited jinja2 templates.

The documentation  is updated for the correct application.

![Screen Shot 2021-06-17 at 23 48 54](https://user-images.githubusercontent.com/24256554/122476543-b3af0c80-cfc6-11eb-943c-7f3a7fc902e8.png)
