Upgrade Theme to a C++ QObject, and unlock customization of default color schemes for custom apps

[kaustuvpokharel]

This PR replaces the legacy Theme.qml singleton with a new C++ Theme singleton that exposes the same theme API to QML (Theme.mainColor, Theme.darkTheme, Theme.applyAppearance(), etc), while loading theme values from a JSON file

The goal was to keep QML usage as unchanged as possible while moving theme logic to a more scalable and maintainable engine

What changed

• Removed the old QML-based Theme.qml singleton
• Added a C++ Theme QObject singleton that mirrors the previous QML API
• Theme colors are now loaded from :/theme/theme.json
• Introduced support for:
  • darkThemeColors
  • lightThemeColors
  • as many theme can be added or extended to, one change in json reflects through entire application.
• Implemented dynamic palette application via Qt meta-object system:
  • applyColors(QVariantMap)
  • applyAppearance(extraColors, baseAppearance)

Reason

The old Theme.qml worked well, but to overcome these:

• theme values were hardcoded in QML
• scaling to more themes / white-label builds was difficult for customization

The new C++ implementation gives us:

• JSON-driven theming (easy to customize / brand / extend)
• centralized theme engine logic
• better long-term maintainability
• a cleaner path for white-label and runtime theme customization
• compatibility with the existing QML codebase (same property names and theme calls)

Compatibility / Migration

This PR is designed to be a drop-in replacement for most existing QML usage.
Existing QML calls like:

• Theme.mainColor
• Theme.mainTextColor
• Theme.darkTheme
• Theme.applyAppearance()
• Theme.getThemeIcon(...)
  continue to work through the new C++ singleton.
  Additionally, darkThemeColors and lightThemeColors are exposed for compatibility with QML code that reads the palette maps directly.

Implementation details

• Switchable colors (dark/light) are applied dynamically via applyColors() using Qt’s meta-object system
• Fixed colors are loaded once from fixedColors in JSON and assigned directly
• errorColor remains derived from darkTheme
• Font helpers and icon path helpers from the previous QML singleton are preserved

Material.theme / Application.styleHints.colorScheme remain QML-side concerns and should be bound to Theme.darkTheme as needed
This PR creates a foundation for future improvements like (if needed or wished):

• multiple theme packs
• white-label theme switching
• runtime theme overrides
• font configuration from JSON (future extension)

A demo of minor tweak in the json transforming whole qfield, (not intended to disturb branding just a test run):

Screen.Recording.2026-02-22.at.10.55.08.PM.mov

[qfield-fairy]

📱 Android builds

Download an Android arm64 build of this PR for testing.
(Built from commit 36f204e)

Other Android architectures

• arm-neon
• x64
• x86
• all access arm64
• all access arm-neon
• all access x64
• all access x86

🍎 MacOS DMG universal builds

Download a MacOS DMG universal build of this PR for testing.
(Built from commit 36f204e)

🪟 Windows builds

Download a Windows build of this PR for testing.
(Built from commit 36f204e)

🐧 Linux AppImage builds

Download a Linux AppImage build of this PR for testing.
(Built from commit 36f204e)

[github-actions]

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