Notes widget: semi-transparent background

[solitone]

I'm in plasma 5 and I'm customizing the breeze dark theme. In my 
metadata.desktop file I've got the following values in the ContrastEffetct 
section:

[ContrastEffect]
enabled=true
contrast=0.45
intensity=0.45
saturation=1.7

Thanks to them, the panel, the menu, notifications, etc. are not completely 
transparent, so readability is improved.

How can I apply such parameters to the Notes widget as well? Currently, if I 
choose a translucent background, the widget becomes completely transparent. 
Moreover, f I put a notes.svg that seems semi-transparent in my theme 
directory, the Notes widget becomes completely transparent, irrespective of 
what background color I choose for it.

Thanks & Regards, 
  Davide