perf: defer dark green theme styles

src/styles/index.css

@@ -6,4 +6,3 @@
 @import "./components/toast.css";
 @import "./components/page-transition.css";
 @import "./components/motion.css";
-@import "./themes/dark-green.css";

src/styles/loadDarkGreenTheme.ts

@@ -0,0 +1,6 @@
+let darkGreenThemePromise: Promise<unknown> | null = null;
+
+export function loadDarkGreenTheme(): Promise<unknown> {
+  darkGreenThemePromise ??= import("./themes/dark-green.css");
+  return darkGreenThemePromise;
+}