Added property to evaulate platform theme mode (iOS, web, Windows)

[YuDavidCao]

Added property to evaulate platform theme mode

• iOS (tested)
• web (tested)
• Windows (yet to be tested)

Fixes #2841

Credit to @Rjvs

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by: Cursor - for understanding the code structure

[freakboy3742]

Thanks for the PR.

The Winforms backend won't be accurate, because .NET 4 support for dark mode doesn't exist. However, AFAICT this will expose the user's preference for dark mode, which should be accurate, and could be manually reflected by the user, so it's better than the current 🤷 API response.

The only issue I'm seeing with this PR is a coverage gap - this is the source of the two Windows failures. The implication is that the key isn't set on the two test machines (which seems plausible - there's no reason for a dark mode preference to be set on a CI machine). We might need to mark that entire branch "no-cover" since we can't reliably test the outcome (if only because we don't know how the end-user's machine is configured.)

[freakboy3742]

Merging with the 2 CI failures; those are known issues (#4415 and #4427) that I'm currently working on.