Feature Request: Support prefers-color-scheme and Default Theme Selection

I would love to see FluentCommunity support the browser's native prefers-color-scheme setting, along with an option to define a default theme for new users.

Why this would be useful

Currently, if a community wants Dark Mode to be the default experience, the available solutions rely on custom JavaScript snippets and localStorage manipulation. While these work, they can introduce issues such as a brief flash of the Light theme before Dark Mode is applied.

Using prefers-color-scheme would provide a much smoother user experience:

• Users who prefer dark interfaces at the operating system/browser level would automatically see Dark Mode.
• Users who prefer light interfaces would automatically see Light Mode.
• No flash of the incorrect theme on first load.
• No custom JavaScript required.
• Better support for PWAs and mobile devices.
• More consistent behavior across browsers and operating systems.

Suggested options

Community Customizer → Theme Settings

Default Theme

• System (prefers-color-scheme)
• Light
• Dark

This would allow community owners to choose whether they want to respect the user's system preference or force a specific default theme for new users.

User-selected preferences could still override the default and continue to be stored normally.

Example Behavior

• System → Follow the user's operating system/browser preference on first visit.
• Dark → New users see Dark Mode by default.
• Light → New users see Light Mode by default.

After a user manually changes the theme, their preference should continue to be remembered and take precedence over the default setting.

I believe this would improve both performance and user experience while eliminating the need for various dark-mode workarounds currently shared in the community.