T1308277 - Fix: Vitest, the "window.getComputedStyle" error is not defined when running tests (#31988)

Author: ajivanyandev

packages/devextreme/js/__internal/ui/themes.ts

@@ -50,6 +50,9 @@ function readThemeMarker(): string | null {
   let result: string;
 
   try {
+    if (!window?.getComputedStyle) {
+      return null;
+    }
     result = window.getComputedStyle(element.get(0)).fontFamily;
     if (!result) {
       return null;

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

@@ -825,3 +825,39 @@ QUnit.module('initialized method', (hooks) => {
         });
     });
 });
+
+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 = undefined;
+
+        try {
+            themes.resetTheme();
+            const value = themes.current();
+            assert.strictEqual(value, null, 'current() returns null on getComputedStyle being undefined');
+        } finally {
+            window.getComputedStyle = originalGetComputedStyle;
+            done();
+        }
+    });
+
+    test('waitForThemeLoad resolves even if getComputedStyle continuously throws', function(assert) {
+        const done = assert.async();
+        const originalGetComputedStyle = window.getComputedStyle;
+        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');
+            window.getComputedStyle = originalGetComputedStyle;
+            themes.setDefaultTimeout(defaultTimeout);
+            done();
+        });
+
+        themes.waitForThemeLoad('some.nonexistent.theme');
+    });
+});