Follow system color scheme by default instead of light

web/src/components/ThemeToggle/ThemeToggle.tsx

@@ -8,7 +8,10 @@ const DARK_THEME = 'dark'
 
 const ThemeToggle = () => {
   const [theme, setTheme] = useState(
-    localStorage.getItem('theme') ?? LIGHT_THEME
+    (localStorage.getItem('theme') ??
+      window.matchMedia('(prefers-color-scheme: dark)').matches)
+      ? DARK_THEME
+      : LIGHT_THEME
   )
 
   const handleToggle = (e: React.ChangeEvent<HTMLInputElement>) => {