fix tests

Author: ajivanyandev

packages/devextreme/testing/tests/DevExpress.ui/themes.tests.js

@@ -830,12 +830,12 @@ QUnit.module('readThemeMarker error handling', () => {
     test('readThemeMarker returns null when getComputedStyle throws an error', function(assert) {
         const done = assert.async();
         const originalGetComputedStyle = window.getComputedStyle;
-        window.getComputedStyle = function() { throw new Error('getComputedStyle fails'); };
+        window.getComputedStyle = undefined;
 
         try {
             themes.resetTheme();
             const value = themes.current();
-            assert.strictEqual(value, null, 'current() returns null on getComputedStyle error');
+            assert.strictEqual(value, null, 'current() returns null on getComputedStyle being undefined');
         } finally {
             window.getComputedStyle = originalGetComputedStyle;
             done();
@@ -845,14 +845,14 @@ QUnit.module('readThemeMarker error handling', () => {
     test('waitForThemeLoad resolves even if getComputedStyle continuously throws', function(assert) {
         const done = assert.async();
         const originalGetComputedStyle = window.getComputedStyle;
-        window.getComputedStyle = function() { throw new Error('boom'); };
+        window.getComputedStyle = undefined;
 
         const TEST_TIMEOUT = 30;
         themes.resetTheme();
         themes.setDefaultTimeout(TEST_TIMEOUT);
 
         themes.ready(() => {
-            assert.strictEqual(themes.current(), null, 'theme remains null after timeout with errors');
+            assert.strictEqual(themes.current(), null, 'theme remains null after timeout');
             window.getComputedStyle = originalGetComputedStyle;
             themes.setDefaultTimeout(defaultTimeout);
             done();