Picture Card - using a theme

I am working on a simple project at the moment to control my TV using PICTURE cards which have RESTfull calls as services. This is all working and I am really happy.

I am using some custom images for the button but the thing I can’t figure out is how to use the themes.

I want to adjust the padding and alignment properties of the card so that my image is centered nicely within the card. Currently the images are top aligned and using the whole width of the card…
I could edit the actual images but that seems a bit silly.

Any help would be appreciated. Thanks.!

I am looking through examples of themes and am now wondering if its only possible to adjust a few params like colours + shadows in themes? ie I won’t be able to adjust other css aspects like margins/borders/etc

Is this corrcet?