Focus notebook find widget when it's shown

Fixes #97204

Fixes #80656

#78168

Readonly helps with type guards and code comprehension

Fixes #59606

This matches how the editor find widget works

Part of #60565

**Problem**
`ITheme.getColor` is one of the major users of the type `null` in our codebase. However this function is used in many places that expected an `undefined` type instead. With the strict null work, this means that we have to write code such as `theme.getColor(...) || undefined` in some cases or have to work with  `Color | null | undefined` types.

**Fix**
`undefined` is generally better supported by TypeScript and more natural in JavaScript. This change makes `ITheme.getColor` return `Color | undefined` instead of `Color | null`.