Update theme detection to use activeColorTheme API

[zetaloop]

Replaced the use of workbench.colorTheme with the newer window.activeColorTheme.kind API.

This change improves compatibility with VSCode's "Window: Auto Detect Color Scheme" setting.
When that setting is enabled, VSCode does not update workbench.colorTheme, but instead uses preferredDarkColorTheme or preferredLightColorTheme internally.
As a result, relying on colorTheme alone may no longer reflect the actual active theme.

The new API correctly reflects the effective color theme at runtime, ensuring PlatformIO Home applies the appropriate light/dark mode on load.

Before

After

[CLAassistant]

All committers have signed the CLA.

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