Make nimble aware of prefers-color-scheme

😯 Problem to Solve

The browser has the prefers-color-scheme media query for letting web applications be aware of the operating system configured light / dark mode setting.

Nimble should be aware of that configuration so nimble applications can automatically adjust to the operating system configured theme preference.

💁 Proposed Solution

• Currently nimble's default theme is the Light theme. So if a user does not provide a <nimble-theme-provider> on the page then the default theme value will be light. We should instead make the default theme be the OS configured theme setting.
• Nimble theme-provider should have an option for the theme property that allows explicitly configuring the OS specified preferred theme. ie theme = light | dark | color | platform

As part of a primeday project I prototyped with adding a platform option to the theme-provider's theme property.

We should still have an HLD to pin down the requirements and API for this change beyond the proposed solution.

📋 Tasks

• HLD

Start with the theme-provider entry point at packages/nimble-components/src/theme-provider/index.ts and review the linked prototype for the proposed platform option. Draft the required HLD to settle the theme API and prefers-color-scheme behavior. Done means the requirements are agreed and the theme provider can represent an explicitly selected or OS-preferred theme.