Changed preview code

assets/hello.rs

@@ -1,6 +0,0 @@
-
-// Hello World
-fn main() {
-    println!("Hello World!");
-}
-

assets/theme_preview.rs

@@ -0,0 +1,5 @@
+// Output the square of a number.
+fn print_square(num: f64) {
+    let result = f64::powf(num, 2.0);
+    println!("The square of {:.2} is {:.2}.", num, result);
+}

src/main.rs

@@ -134,12 +134,13 @@ pub fn list_themes(assets: &HighlightingAssets, config: &mut Config) {
     let themes = &assets.theme_set.themes;
     let mut style = HashSet::new();
     style.insert(OutputComponent::Plain);
-    config.files = vec![Some("assets/hello.rs")];
+    config.files = vec![Some("assets/theme_preview.rs")];
     config.output_components = OutputComponents(style);
     for (theme, _) in themes.iter() {
-        println!("{}", theme);
+        println!("{}\n", theme);
         config.theme = theme.to_string();
         let _controller = Controller::new(&config, &assets).run();
+        println!("--------------------------------\n");
     }
 }