Enable overriding stylesheet for themable components

As a developer
I can override the style of an individual component instance
so I can tweak it as necessary

---

I think we will likely be asked to support this, and I think it's a reasonable thing for teams to expect. That said I have some concerns about it.

My main concern is that it represents a huge new API surface area. The more surface area we expose, the less flexible/more breaky our library gets.

- How might we designate a prop as use-at-your-own-risk?
- What other ways might we support overriding one-off?
- Should we just let them use CSS instead?

_Shotgun does something like this_
```
function myInputStylesheet(stylesheet, props, themeData) {
return {
...stylesheet, input: {
...stylehseet.input,
backgroundColor: props.hasFocus ? "blue" : "transparent"
}
}
}
}

function MyInput(props) {
return
}
```

---

### Dev notes

For reference, A subcomponent of `Flyout` supports this approach and `Tooltip` uses it.

Start by reading the existing stylesheet override approach in the Flyout subcomponent and how Tooltip uses it. Compare those entry points with the proposed per-instance stylesheet function and review the open design questions before choosing a direction. Done requires an agreed API or alternative for one-off component styling, with its scope and tests documented.

Written by the indexing model from the issue text.