Find out CSS shorthands

In this forums I see customization like this. I see that in the api docs I find that ion-content supports --background. But now, how do I know what I cann pass to that and in which order?

ion-content {
    --background: url('assets/imgs/testPic.jpg')  0 0/50% 10% no-repeat;
}

I use and recommend MDN for all my documentation on stuff like this.

Will look into that. Looks like exactly what I wanted. Thanks a lot!