Replace lingering page.url instance with site.baseurl

Refs #12083 & #12037

docs/css.html

@@ -3462,7 +3462,7 @@ Bootstrap.javascripts_path
 @import "bootstrap/theme";
 {% endhighlight %}
 
-    <p>The full list of Bootstrap variables can be found <a href="{{ page.url }}customize/#less-variables">in the Customizer</a>. You can override these by simply redefining the variable before the <code>@import</code> directive, e.g.:</p>
+    <p>The full list of Bootstrap variables can be found <a href="{{ site.baseurl }}customize/#less-variables">in the Customizer</a>. You can override these by simply redefining the variable before the <code>@import</code> directive, e.g.:</p>
 
 {% highlight sass %}
 $navbar-default-bg: #312312;