Nextcloud-release/theme/nextcloud.VisualElementsManifest.xml

<Application xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>
  <VisualElements
	BackgroundColor="#151e3d"
	ForegroundText="light"
    ShowNameOnSquare150x150Logo="on"
    Square150x150Logo="visualelements\150-Nextcloud-w10startmenu.png"
	Square70x70Logo="visualelements\70-Nextcloud-w10startmenu.png"/>
</Application>