how to programatically theme an activity to be like a dialog?

Question

How does one programatically (without touching the AndroidManifext.xml) set the theme of an Activity so that it looks like a dialog?

Note: I am ok with modifying the AndroidManifext.xml as long as it does not need to be modified in order to switch between making it look like a normal activity or a dialog.

What I've tried so far

I tried the following as per this stackoverflow answer:

public class DialogActivity extends Activity
{

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        setTheme(android.R.style.Theme_DeviceDefault_Dialog);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_dialog);
        Log.d(TAG,"Build.VERSION.SDK_INT: "+Build.VERSION.SDK_INT); // 23
    }
}

But it ends up blacking out everything in the background.

I also saw this stackoverflow answer, and tried:

public class DialogActivity extends Activity
{

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        setTheme(android.R.style.Theme_DeviceDefault_Dialog);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_dialog);
        getWindow().setBackgroundDrawable(new ColorDrawable(android.graphics.Color.TRANSPARENT));
    }
}

but it ends up making everything black.

What do? Thank you.

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