Introduce left and right colors for slider rails

style/src/slider.rs

@@ -5,11 +5,26 @@ use iced_core::Color;
 #[derive(Debug, Clone, Copy)]
 pub struct Appearance {
     /// The colors of the rail of the slider.
-    pub rail_colors: (Color, Color),
+    pub rail: Rail,
     /// The appearance of the [`Handle`] of the slider.
     pub handle: Handle,
 }
 
+/// The appearance of a slider rail
+#[derive(Debug, Clone, Copy)]
+pub struct Rail {
+    /// The colors of the rail of the slider.
+    pub colors: (Color, Color),
+    /// The height of a slider rail and the width of a vertical slider.
+    pub size: f32,
+    /// The border radius of the slider.
+    pub border_radius: f32,
+    /// The border width of the slider.
+    pub border_width: f32,
+    /// The border [`Color`] of the slider.
+    pub border_color: Color,
+}
+
 /// The appearance of the handle of a slider.
 #[derive(Debug, Clone, Copy)]
 pub struct Handle {

style/src/theme.rs

@@ -416,10 +416,16 @@ impl slider::StyleSheet for Theme {
                 };
 
                 slider::Appearance {
-                    rail_colors: (
-                        palette.primary.base.color,
-                        Color::TRANSPARENT,
-                    ),
+                    rail: slider::Rail {
+                        colors: (
+                            palette.primary.base.color,
+                            palette.primary.base.color,
+                        ),
+                        size: 2.0,
+                        border_radius: 0.0,
+                        border_width: 0.0,
+                        border_color: Color::TRANSPARENT,
+                    },
                     handle: slider::Handle {
                         color: palette.background.base.color,
                         border_color: palette.primary.base.color,