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/ui/src/components/button/button_like.rs

@@ -202,7 +202,12 @@ impl ButtonStyle {
                     icon_color: Color::Default.color(cx),
                 }
             }
-            ButtonStyle::Tinted(tint) => tint.button_like_style(cx),
+            ButtonStyle::Tinted(tint) => {
+                let mut styles = tint.button_like_style(cx);
+                let theme = cx.theme();
+                styles.background = theme.darken(styles.background, 0.05, 0.2);
+                styles
+            }
             ButtonStyle::Subtle => ButtonLikeStyles {
                 background: cx.theme().colors().ghost_element_hover,
                 border_color: transparent_black(),