[theme demo] Fix dropdown losing choice on solarized light / dark

Choosing "solarized dark" correctly sets .cm-s-solarized .cm-s-dark
(as documented https://codemirror.net/doc/manual.html#option_theme).

It then sets URL fragment to #solarized%20dark,
which was looking for `solarized%20dark` in dropdown and failing.
This commit makes both setting and getting URL fragment reliable.

Author: cben

demo/theme.html

@@ -183,17 +183,18 @@ <h2>Theme Demo</h2>
   function selectTheme() {
     var theme = input.options[input.selectedIndex].textContent;
     editor.setOption("theme", theme);
-    location.hash = "#" + theme;
+    location.hash = "#" + encodeURIComponent(theme);
   }
-  var choice = (location.hash && location.hash.slice(1)) ||
+  var choice = (location.hash &&
+                decodeURIComponent(location.hash.slice(1))) ||
                (document.location.search &&
                 decodeURIComponent(document.location.search.slice(1)));
   if (choice) {
     input.value = choice;
     editor.setOption("theme", choice);
   }
   CodeMirror.on(window, "hashchange", function() {
-    var theme = location.hash.slice(1);
+    var theme = decodeURIComponent(location.hash.slice(1));
     if (theme) { input.value = theme; selectTheme(); }
   });
 </script>