Transparent tooltip when using page as tooltip

Hello,

i'm adding contextual information to my dashboard using the method described here. 

Is there a way to make the tooltip a bit transparent, it would look much neater. I've already set the text boxes/shapes i'm using on the tooltip page as transparent and checked that the canvas is transparent, even set the tooltip setting of the blank KPI card to higher transparency (it does nothing anyway..).

Maybe someone has a trick?!

Best regards