Light/dark theming

[Alex2772]

A solution is needed for light/dark theming. It's probably depends on #572.

• If possible, light/dark theme of system UI elements should be implemented as well. (Themed titlebar on Srindows 10+ #675)

API sketch

namespace aui::theme {


// 1. enum
enum class Theme { // i don't like this name, just for sake of demo
  LIGHT,
  DARK,
  HIGH_CONTRAST, // etc...
};

// 2. this reflects setting in OS settings
// Windows: Settings -> System -> Theme -> Dark/light
// Linux: depends on DE
const AProperty<Theme>& systemPreference();

// 3. main observable for theme value
//
// this is up to be defined by AUI-based application.
// by default, we'll return light theme because the application
// developer might not know/care about light/dark themes.
//
// if they want, they'll override application preference to
//
//  appPreference = [] { return *aui::theme::systemPreference(); }
//
// or, if they want to introduce their application-specific setting,
//
// appPreference = [] {
//   // assuming AProperty<AOptional<Theme>> theme
//   if (MyAppSettings::theme->hasValue()) {
//     return MyAppSettings::theme->value();
//  }
//  return *systemPreference();
// }
APropertyPrecomputed<Theme> appPreference = [] { return Theme::LIGHT; }
//
// this value should be observed in entire application. If AUI can adjust system UI elements
// (i.e., titlebars #675), appPreference will affect them too.
}


// 4. Assuming #572 is resolved (at least partially)
// AUI_OVERRIDE_STYLE implicitly subscribes for aui::theme::appPreference changes
static _<AView> myThemeAwareButton() {
  return Button { ... } AUI_OVERRIDE_STYLE {
    BackgroundSolid(aui::theme::appPreference == Theme::LIGHT ? AColor::WHITE : AColor::BLACK),
  };
}