Implement theme variant auto-switching (dark/light) for Windows

[dweymouth]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

There is currently no implementation of detecting the dark/light preference changing on Windows. This was previously only tracked with a TODO in the code here

Is it possible to construct a solution with the existing API?

No response

Describe the solution you'd like to see.

An implementation for Windows, possibly based on this MIT-licensed Gist

[dweymouth]

Fixed on release branch for 2.5.1