How can I fix this error: You need to use a Theme.AppCompat theme (or descendant) with this activity

If you have another styles files in side another values folders like "values-v11", "values-v14"... Edit theme also and try to clean your app before running.

Edited: From your activity change getApplicationContext() to this:

AlertDialog.Builder builder = new AlertDialog.Builder(getApplicationContext());

to

AlertDialog.Builder builder = new AlertDialog.Builder(this);

Because the dialog also should extends the Appcompat Theme.