Grayout Background On/Off setting in 'Reference Label Layout' for a KPI Card

Hi Community Members,

I am a new Power BI user and currently having a difficult time setting the background of my KPI card to transparency before I can place it on a black rectangle. 

I have turned off all the background options in Visualization pane except 'Reference label Layout' section where the background is ON by default and cannot be turned off since the field grays out. I am not sure whether it is the root cause. I have attached a photo of this issue here.

Please kindly advise me how to make the transparent KPI card background if you have come across similar experience.

Thanks!

Sunlong