Add Theming Support to MAUI Expert

[melgoharyme]

Description

Right now, the MAUI Expert isn’t handling light/dark mode or custom themes consistently. Developers might see controls or pages using the wrong colors or styles, which can be confusing.

It would be great to update the Expert so it:

• Knows how to detect the current system theme.
• Can apply custom themes properly to controls and pages.
• Updates the UI automatically when the theme changes.