Why can't the button be set size and other styles?

from PyQt5.QtWidgets import *
from PyQt5.QtCore import QSize

import sys
from PyQt5 import QtCore, QtGui, QtWidgets
class Ui_Form(object):
    def setupUi(self, Form):
        Form.setObjectName("Form")
        Form.resize(345, 225)
        self.pushButton = QtWidgets.QPushButton(self)
        self.pushButton.setGeometry(QtCore.QRect(20, 30, 75, 23))
        self.pushButton.setObjectName("pushButton")
        self.retranslateUi(Form)
        QtCore.QMetaObject.connectSlotsByName(Form)

    def retranslateUi(self, Form):
        _translate = QtCore.QCoreApplication.translate
        Form.setWindowTitle(_translate("Form", "Form"))
        self.pushButton.setText(_translate("Form", "PushButt"))

class MyButton(QPushButton):
    def __init__(self):
        super().__init__()
        self.setStyleSheet("background:green")  #Writing this way will not produce practical results
        self.setFixedSize(QSize(10,10)) #Writing this way will not produce practical results

class Demo(Ui_Form,QWidget):
    def __init__(self,parent=None):
        super().__init__(parent)
        self.setupUi(self)
        self.btn = MyButton()
        self.btn.setFixedSize(QSize(100,20))  #Writing this way will not produce practical results

if __name__ == '__main__':
    app = QApplication([])
    w = Demo()
    w.show()
    sys.exit(app.exec_())