Light/dark theming

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. (#675)

## API sketch

```cpp
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& 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> theme
// if (MyAppSettings::theme->hasValue()) {
// return MyAppSettings::theme->value();
// }
// return *systemPreference();
// }
APropertyPrecomputed 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 _ myThemeAwareButton() {
return Button { ... } AUI_OVERRIDE_STYLE {
BackgroundSolid(aui::theme::appPreference == Theme::LIGHT ? AColor::WHITE : AColor::BLACK),
};
}
```

Start with the API sketch in this issue and review dependencies #572 and #675, including the system UI theming requirement. Define how application and system preferences should be observed across supported platforms, then verify that theme-aware styles update and that the requested system UI behavior is addressed where possible.

Written by the indexing model from the issue text.