feat: Added light and dark features

- Updated slint to 1.15.1

build.rs

@@ -1,6 +1,18 @@
 fn main() {
+    
+    let mut style = String::from("cosmic");
+
+    #[cfg(all(feature = "dark", feature = "light"))]
+    compile_error!("Features \"dark\" and \"light\" are mutually exclusive");
+
+    if cfg!(feature = "dark") {
+        style.push_str("-dark");
+    } else if cfg!(feature = "light") {
+        style.push_str("-light");
+    }
+
     let config = slint_build::CompilerConfiguration::new()
-        .with_style("cosmic".into());
+        .with_style(style);
 
     slint_build::compile_with_config("ui/app-window.slint", config).expect("Slint build failed");
 }