feat: add native material backgrounds

crates/theme/src/colors.rs

@@ -3,6 +3,8 @@
 
 use gpui::{Hsla, hsla, rgb};
 
+use crate::ThemeVariant;
+
 fn hex(hex: u32) -> Hsla {
     rgb(hex).into()
 }
@@ -115,3 +117,44 @@ impl ColorTokens {
         }
     }
 }
+
+/// The translucent paints surfaces lay over a native window material.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct MaterialTokens {
+    /// The window-sized fill the material shows through.
+    pub background: Hsla,
+    /// The hover wash over material surfaces.
+    pub hover: Hsla,
+    /// The selected-row fill over material surfaces.
+    pub selection: Hsla,
+}
+
+impl MaterialTokens {
+    /// Derives the material paints from the resolved palette.
+    pub(crate) fn derive(
+        colors: &ColorTokens,
+        selection: Hsla,
+        variant: ThemeVariant,
+        translucent: bool,
+    ) -> Self {
+        if !translucent {
+            return Self {
+                background: colors.background,
+                hover: colors.muted,
+                selection,
+            };
+        }
+
+        // Light tints heavier than dark: the blur carries less contrast there.
+        let background_alpha = match variant {
+            ThemeVariant::Light => 0.6,
+            ThemeVariant::Dark => 0.5,
+        };
+        Self {
+            background: colors.background.alpha(background_alpha),
+            // A foreground wash keeps hover visible over any backdrop hue.
+            hover: colors.foreground.alpha(0.06),
+            selection: selection.alpha(0.85),
+        }
+    }
+}