Shape/Background of MiniPlayer

[Bassmurf]

Hi, I love the miniplayer, and have checked a lot to see how to do. However, Im stuck now as I want to change the shape of the miniplayer: not in a complete different way, instead of a rectangle, i want a rounded rectangle. Creating the rounded rectangle was easy, just putting in the miniplayer builder a rounded rectangle and thats it.

However, by doing so I saw that there is a background of the miniplayer, where my container is put on top. In the following picture, you can see a red rounded rectangle, thats fine, but in the corners you have black parts (which I figured was the color from my app theme).

So i thought i could put the theme to transparent, but it didnt work, still showed a black rectangle under. By putting the theme to white, is showed a white one. I tried using the backgroundcolor of the miniplayer, but it only changes the color of the fading effect over other widgets when expanding the player. So how can i make the miniplayer background dissepear? Just a backgroundcolor for it so I can put Colors.transparent would work and we could make most shapes we want. Please help