Extend Theming: Documentation and samples

[PhilJollans]

I am interested in ability to extend theming, as mentioned in
https://primeng.org/theming#extend

My knowledge level is not good enough to get started based on the information on that page.
Is there more documentation of extending the theming which I have not found?
Is there sample code?

In our project, we are using validations to change the style of some input elements, for example:

Currently this is done using additional styles, which is a bit messy.
This seems like the kind of thing which could be handled by extending the Theming.

Of course, if there is a better way, I am open to suggestions.

Phil