My Android Studio 2.1.1 (Windows XP) is giving trouble when it is opened: the virtual cell phone screen is gray instead of white. With this, any item that I enter in my project is invisible.
I even found the solution:
Click the "AppTheme" button, located above the preview, and in the "Select Theme" dialog box, choose another pro app theme: "Material Dark", "Material Light", "Holo Dark", Holo Light "etc.