Draft first-class Theme support

RFC: https://github.com/iced-rs/rfcs/pull/6

style/src/theme.rs

@@ -0,0 +1,45 @@
+use crate::button;
+
+use iced_core::{Color, Vector};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Theme {
+    Light,
+    Dark,
+}
+
+impl Default for Theme {
+    fn default() -> Self {
+        Self::Light
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Button {
+    Primary,
+    Secondary,
+    Positive,
+    Destructive,
+    Text,
+}
+
+impl Default for Button {
+    fn default() -> Self {
+        Self::Primary
+    }
+}
+
+impl button::StyleSheet for Theme {
+    type Variant = Button;
+
+    fn active(&self, _variant: Self::Variant) -> button::Style {
+        button::Style {
+            shadow_offset: Vector::default(),
+            background: None,
+            border_radius: 0.0,
+            border_width: 0.0,
+            border_color: Color::TRANSPARENT,
+            text_color: Color::BLACK,
+        }
+    }
+}