Use a custom OpenLayers theme

Instead of having a customised copy of the default theme in the
app assets, use a custom theme that pulls in the default one and
then overrides things as needed.

app/assets/javascripts/map.js.erb

@@ -19,7 +19,7 @@ function createMap(divName, options) {
       ],
       numZoomLevels: 20,
       displayProjection: new OpenLayers.Projection("EPSG:4326"),
-      theme: "<%= asset_path 'theme/default/style.css' %>"
+      theme: "<%= asset_path 'theme/openstreetmap/style.css' %>"
    });
 
    var mapnik = new OpenLayers.Layer.OSM.Mapnik(i18n("javascripts.map.base.standard"), {