Draft first-class Theme support

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

examples/integration_wgpu/src/main.rs

@@ -188,6 +188,7 @@ pub fn main() {
                             viewport.scale_factor(),
                         ),
                         &mut renderer,
+                        &iced_wgpu::Theme::Dark,
                         &mut clipboard,
                         &mut debug,
                     );