use os detection for initial dark mode setting

src/utils/os.ts

@@ -3,6 +3,22 @@
 
 // Utility functions for dealing with operating systems.
 
+/**
+ * Tests if we are running on macOS.
+ * @returns `true` if running on macOS, otherwise `false`.
+ */
 export function isMacOS(): boolean {
     return /mac/i.test(navigator.platform);
 }
+
+/**
+ * Tests if the OS is set to dark mode.
+ * @returns: `true` if dark mode should be preferred, otherwise `false`.
+ */
+export function prefersDarkMode(): boolean {
+    if (!window.matchMedia) {
+        return false;
+    }
+
+    return window.matchMedia('(prefers-color-scheme: dark)').matches;
+}