Add Dark Theme Support in Cupertino Design

[jsuarezruiz]

Apple iOS controls: https://developer.apple.com/design/human-interface-guidelines/ios/controls/buttons/

What should you do?

• Use the Figma file to validate the correct colors, etc: https://www.figma.com/community/file/857576771468552929
• Use Application.Current?.RequestedTheme and other Color methods extensions to apply the correct color when the system or app theme is Dark.
• Send a PR with the changes and share some screenshots!

You can see the Material Dark theme implementation here: https://github.com/dotnet/Microsoft.Maui.Graphics.Controls/pull/40