Set dark mode by default

src/main.rs

@@ -1,6 +1,6 @@
 use iced::{
     widget::{column, text, text_editor},
-    Element, Result, Sandbox, Settings,
+    Element, Result, Sandbox, Settings, Theme,
 };
 
 #[derive(Debug, Clone)]
@@ -40,6 +40,10 @@ impl Sandbox for Application {
             .padding(10)
             .into()
     }
+
+    fn theme(&self) -> Theme {
+        Theme::Dark
+    }
 }
 
 fn main() -> Result {