Add dark theme

It uses prefers-color-scheme CSS @media selector to determine which
theme is used.

Support table: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme#browser_compatibility

static/style.css

@@ -57,3 +57,18 @@ body {
   /* Stretch to form width */
   width: 100%;
 }
+
+@media screen and (prefers-color-scheme: dark) {
+  body {
+    background-color: #212529;
+    color: #f8f9fa;
+  }
+
+  .form__input,
+  .form__button,
+  .textarea {
+    background-color: #131618;
+    border-color: #495057;
+    color: #f8f9fa;
+  }
+}