theme: Add color darken function (#20746)

This PR adds a `darken` function that allows to reduce the lightness of
a color by a certain factor. This popped up as I wanted to add hover
styles to tinted-colors buttons.

Release Notes:

- N/A

crates/theme/src/theme.rs

@@ -315,6 +315,22 @@ impl Theme {
     pub fn window_background_appearance(&self) -> WindowBackgroundAppearance {
         self.styles.window_background_appearance
     }
+
+    /// Darkens the color by reducing its lightness.
+    /// The resulting lightness is clamped to ensure it doesn't go below 0.0.
+    ///
+    /// The first value darkens light appearance mode, the second darkens appearance dark mode.
+    ///
+    /// Note: This is a tentative solution and may be replaced with a more robust color system.
+    pub fn darken(&self, color: Hsla, light_amount: f32, dark_amount: f32) -> Hsla {
+        let amount = match self.appearance {
+            Appearance::Light => light_amount,
+            Appearance::Dark => dark_amount,
+        };
+        let mut hsla = color;
+        hsla.l = (hsla.l - amount).max(0.0);
+        hsla
+    }
 }
 
 /// Compounds a color with an alpha value.