Introduce StyleSheet for Text widget

style/src/text.rs

@@ -0,0 +1,18 @@
+use iced_core::Color;
+
+pub trait StyleSheet {
+    type Style: Default + Copy;
+
+    fn appearance(&self, style: Self::Style) -> Appearance;
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct Appearance {
+    pub color: Option<Color>,
+}
+
+impl Default for Appearance {
+    fn default() -> Self {
+        Self { color: None }
+    }
+}