Add docs on customizing brand

Author: samselikoff

tests/dummy/app/pods/docs/quickstart/template.md

@@ -147,3 +147,14 @@ windows.
 3. **Better scrolling.** You may want to install
 [Ember Router Scroll](https://github.com/dollarshaveclub/ember-router-scroll)
 to enable "scroll to top with preserved browser history scroll position".
+
+4. **Optionally customize your brand.** The primary brand color is <span class='docs-text-brand'>`#E04E39`</span>. You can change this value by setting the `--brand-primary` CSS custom property in your dummy app's `app.css` file.
+
+  For example, to set your brand color as `#3490DC`
+
+  ```css
+  /* tests/dummy/app/styles/app.css */
+  :root {
+    --brand-primary: #3490DC;
+  }
+  ```