QtDesigner: Make MatplotlibWidget Grey Border transparent

how do I make the grey border around a MatplotlibWidget enter image description here

transpartent? Preferably directly in QT Designer. I tried the Stylesheet but it did not change anything.

A little example here: The qt designer file only has the matplotlib widget instantiated.

 <?xml version="1.0" encoding="UTF-8"?>
    <ui version="4.0">
     <class>Form</class>
 <widget class="QWidget" name="Form">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>548</width>
    <height>425</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Form</string>
  </property>
  <widget class="MatplotlibWidget" name="mplwidget">
   <property name="geometry">
    <rect>
     <x>60</x>
     <y>50</y>
     <width>400</width>
     <height>300</height>
    </rect>
   </property>
  </widget>
 </widget>
 <customwidgets>
  <customwidget>
   <class>MatplotlibWidget</class>
   <extends>QWidget</extends>
   <header>matplotlibwidget</header>
  </customwidget>
 </customwidgets>
 <resources/>
 <connections/>
</ui>

changing the style sheet

<widget class="MatplotlibWidget" name="mplwidget">
   <property name="geometry">
    <rect>
     <x>60</x>
     <y>50</y>
     <width>400</width>
     <height>300</height>
    </rect>
   </property>
   <property name="styleSheet">
    <string notr="true">background-color: rgb(255, 255, 255);</string>
   </property>
  </widget>

did not change anything