Use Style struct pattern instead of trait for all widgets

widget/src/vertical_slider.rs

@@ -54,7 +54,7 @@ pub struct VerticalSlider<'a, T, Message, Theme = crate::Theme> {
     on_release: Option<Message>,
     width: f32,
     height: Length,
-    style: fn(&Theme, Status) -> Appearance,
+    style: Style<Theme>,
 }
 
 impl<'a, T, Message, Theme> VerticalSlider<'a, T, Message, Theme>
@@ -75,7 +75,7 @@ where
     ///   `Message`.
     pub fn new<F>(range: RangeInclusive<T>, value: T, on_change: F) -> Self
     where
-        Theme: Style,
+        Style<Theme>: Default,
         F: 'a + Fn(T) -> Message,
     {
         let value = if value >= *range.start() {
@@ -100,7 +100,7 @@ where
             on_release: None,
             width: Self::DEFAULT_WIDTH,
             height: Length::Fill,
-            style: Theme::style(),
+            style: Style::default(),
         }
     }
 
@@ -136,10 +136,7 @@ where
     }
 
     /// Sets the style of the [`VerticalSlider`].
-    pub fn style(
-        mut self,
-        style: impl Into<fn(&Theme, Status) -> Appearance>,
-    ) -> Self {
+    pub fn style(mut self, style: fn(&Theme, Status) -> Appearance) -> Self {
         self.style = style.into();
         self
     }
@@ -357,7 +354,7 @@ where
         let bounds = layout.bounds();
         let is_mouse_over = cursor.is_over(bounds);
 
-        let style = (self.style)(
+        let style = (self.style.0)(
             theme,
             if state.is_dragging {
                 Status::Dragged