redPosci.py: fixed stylesheet to reset color to default

redPosci.py

@@ -497,7 +497,7 @@ class OscDisplay(QWidget, Ui_OscDisplay):
 
     def set_gui4start(self):
         self.startButton.setText("Start")
-        self.startButton.setStyleSheet("color: black")
+        self.startButton.setStyleSheet("")
         self.startButton.clicked.disconnect()
         self.startButton.clicked.connect(self.start)
         self.startDAQButton.setEnabled(True)