theme: build out the theme engine core with resolve pipeline

crates/theme/src/source.rs

@@ -0,0 +1,28 @@
+//! Platform adapters behind one seam: `snapshot()` reads the running
+//! system, `install_watchers()` forwards real change events.
+
+use gpui::AsyncApp;
+
+use crate::ThemeVariant;
+use crate::system::SystemTheme;
+
+pub(crate) trait ThemeSource: 'static {
+    fn snapshot(&self, variant: ThemeVariant) -> SystemTheme;
+    fn install_watchers(&mut self, _async_cx: AsyncApp) {}
+    fn label(&self) -> &'static str;
+}
+
+#[cfg(target_os = "macos")]
+pub(crate) fn platform_source() -> Box<dyn ThemeSource> {
+    Box::new(macos::MacosSource::default())
+}
+
+#[cfg(not(target_os = "macos"))]
+pub(crate) fn platform_source() -> Box<dyn ThemeSource> {
+    Box::new(fallback::FallbackSource)
+}
+
+#[cfg(target_os = "macos")]
+pub(crate) mod macos;
+
+pub(crate) mod fallback;