How to disable MPRIS widget transparent background?

[bonkie25]

I want my mpris widget to have only the album icon and no transparent background to it, as shown in this image from the config flex page in this github page.

I've tried looking this up and the details I found were that you need to set the opacity to 1 or change the rgba value to be (x,y,z,1) however this hasn't changed anything for me at all. It just stays the exact same (pictured below) and lowering the opacity just does as you would expect and make the widget harder to see. How can I set it so it looks like in the image above?

[Where I found out about the opacity thing.]