[UI] White system title bar in Windows 11 Dark Mode

[VicDavis]

What's wrong

When system-wide Dark Mode is enabled in Windows 11, the app's internal UI correctly uses the dark theme, but the native OS title bar remains glaringly white.

How to reproduce

1. Set Windows 11 system color mode to "Dark" (Settings -> Personalization -> Colors).
2. Launch GitHub Store.
3. Look at the top window border (title bar) — it stays white.

Your setup

App version: 1.8.3
Desktop OS: Windows 11

Anything else

Please consider enabling a custom borderless titlebar or using DWMWA_USE_IMMERSIVE_DARK_MODE for the Compose Window to match the system's dark theme.

[rainxchzed]

Great feedback! We will work on it 👍