Fixing "Cannot resolve symbol Theme" error in Android Studio

If you have ever encountered unusual "cannot resolve symbol" messages with regards to Android resource XML files (an example is the screenshot below), which cannot be resolved by running Android Studio's Build | Clean Project or Build | Rebuild Project menu commands, then fixing the issue is very simple.

To resolve the problem, click on the Sync Project with Gradle Files icon in Android Studio, as shown in the screenshot below, or select Tools | Android | Sync Project with Gradle Files. 

Processing files will appear and at the end of it, the error messages no longer appear, as shown below.