Fix ol.css for some examples

- es2015-custom-element needs css variables declared in :host
- external-map-map still does not work when editing in codesandbox,
  though that neither worked before.

site/src/download/index.hbs

@@ -18,7 +18,7 @@ layout: default.hbs
       <p>
         If you want to try out OpenLayers without downloading anything (<b>not recommended for production</b>), include the following in the head of your html page:
 <pre><code class="language-html">&lt;script src="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/{{ version }}/build/ol.js"&gt;&lt;/script&gt;
-&lt;link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/{{ version }}/css/ol.css"&gt;
+&lt;link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/{{ version }}/theme/ol.css"&gt;
 </code></pre>
       </p>
     </div>