Added UI test for themes

This test enables the example theme and checks that it works.

plugins/ExampleTheme/stylesheets/theme.less

 @theme-fontFamily-base: Verdana, sans-serif;
 
-@theme-color-background-base: #b8ffe3;
-@theme-color-brand: #70cad4;
+@theme-color-brand: #5793d4;
+@theme-color-background-base: #d9e0e3;
 
 /*
 @theme-color-brand:                    #d4291f;

tests/UI/specs/Theme_spec.js

+/*!
+ * Piwik - free/libre analytics platform
+ *
+ * Tests that theming works.
+ *
+ * @link http://piwik.org
+ * @license http://www.gnu.org/licenses/gpl-3.0.html GPL v3 or later
+ */
+describe("Theme", function () {
+    this.timeout(0);
+
+    before(function () {
+        testEnvironment.pluginsToLoad = ['ExampleTheme'];
+        testEnvironment.save();
+    });
+
+    it("should use the current theme", function (done) {
+        expect.screenshot("load").to.be.capture(function (page) {
+            page.load("");
+        }, done);
+    });
+});