DiagramContainer background-color/opacity default value

[ericbaril72]

How can I force the background-color/opacity of the diagram background ?
I want to use draw.io to OVERLAY content on top of a in my webapp.

current HTML:

<iframe style="height=600 width=600;position:absolute;top:-600" src="drawio.io/lightbox=1#U"></iframe>

Tried :
iframe style="background-color:none"
--> can't see the canvas underneath

iframe style="opacity:50%"
--> it fades the background but fades the overlay elements as well

Within drawio
The XML-editor top tag lists background='none'
also tried applying low opacity background color

Drawio converts the XML into SVG elements within a .geDiagramContainer

Using Chrome development inspection tool, I manually locate the first within <div.geDiagramContainer> and manually erase background-color: rgb(255, 255, 255) -> TADAM
=> transparent diagram background with all drawn elements on top of my canvas.

BUT, the SVG within geDiagramContainer is auto-generated and the draw.io iframe uses it's own grapheditor.css.

Any suggestions ?