if you want the original style of your Ui to remain and the title bar to be removed with no effect on that, you have to remove the title bar in your activity rather than the manifest. leave the original theme style that you had in the manifest and in each activity that you want no title bar use this.requestWindowFeature(Window.FEATURE_NO_TITLE); in the oncreate() method before setcontentview() like this:
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
this.requestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.activity_signup);
...
}