theme: build out the theme engine core with resolve pipeline

crates/theme/src/source/fallback.rs

@@ -0,0 +1,20 @@
+//! The adapter for unknown systems:
+//! no colors of its own, resolution degrades to the neutral presets.
+
+use super::ThemeSource;
+use crate::ThemeVariant;
+use crate::system::SystemTheme;
+
+// Constructed only by `platform_source` on non-macOS targets.
+#[cfg_attr(target_os = "macos", allow(dead_code))]
+pub(crate) struct FallbackSource;
+
+impl ThemeSource for FallbackSource {
+    fn snapshot(&self, variant: ThemeVariant) -> SystemTheme {
+        SystemTheme::fallback(variant)
+    }
+
+    fn label(&self) -> &'static str {
+        crate::system::SourceKind::Fallback.label()
+    }
+}