feat: restore custom theme configuration once Terminal.Gui MEC Phase A2 lands

[oca-agent]

Description

With the migration from the obsolete ConfigurationManager to Terminal.Gui's modern Microsoft.Extensions.Configuration (TuiConfigurationBuilder / MEC) setup, the ability for users to specify custom themes via the legacy Themes JSON dictionary in ~/.config/smoc/config.json was removed.

Current State

• Terminal.Gui v2's modern MEC setup currently binds theme selection as a scalar value (ThemeSettings.Theme), while application-specific color schemes are registered programmatically via SchemeManager.AddScheme in AppSchemes.RegisterDefaultSchemes().
• Full JSON-based custom theme and scheme dictionary configuration is planned upstream in Terminal.Gui as part of MEC Phase A2 (see MecThemeManager source notes).

Next Steps / Action Items

1. Track Terminal.Gui upstream releases for MEC Phase A2 / theme dictionary binding support.
2. Re-enable custom theme definition support in ~/.config/smoc/config.json once available.
3. Update README.md with documentation and configuration examples for custom themes.