Dark mode.

components/settings/ThemeButton.js

@@ -0,0 +1,13 @@
+import React from 'react';
+import Button from 'components/common/Button';
+import useTheme from 'hooks/useTheme';
+
+export default function ThemeButton() {
+  const [theme, setTheme] = useTheme();
+
+  function handleClick() {
+    setTheme(theme === 'light' ? 'dark' : 'light');
+  }
+
+  return <Button onClick={handleClick}>{theme}</Button>;
+}