adding document mode

I've been thinking a lot about the treehouse and I feel like it's time to say goodbye to the tree format.

static/css/base.css

@@ -1,4 +1,4 @@
-/* Color scheme. */
+/* Color scheme */
 
 :root {
     --accent-red: #fb4c9e;
@@ -36,6 +36,18 @@
     }
 }
 
+/* Animations */
+
+:root {
+    --transition-duration: 0.15s;
+}
+
+@media (prefers-reduced-motion: reduce) {
+    :root {
+        --transition-duration: 0;
+    }
+}
+
 /* Reset things to more sensible sizing rules */
 
 * {