How to deal with picture aligninment and background color?

[jackstuard]

First, thank you for your plugin; it works great here!

One thing that I could not do, and by looking at your great examples, I saw that you could centralize and make all the pictures look awesome (like full screen). Even with the "fit_mode: contain," there are times when I see a white background.

I tried to change the background to black with card_mod, but on PC, it shows the background changed; on Hub Max, no.
Another thing is trying to centralize the picture in the middle. Do you have success with that?