'floating' elements and background image

Ionic allows to create different elements but mostly they are aligned to the** left, right or center… **
there are some apps whose elements are ‘floating’ in the screen, let say a picture that covers the whole screen, even a video running in the background… and then elements positioning inside.
how do you manage the elements to be ‘center’ vertically or horizontally using Ionic? is this a CSS + javascript calculation or is there a way to do it easier? how to get this to work with multiple devices with different sizes?
iOS (xcode) let you design those ‘hooks’ to aling the elements ‘referenced’ to the edges of the device so it will reposition depending on the size, rotation, etc… how Ionics handles this?

Same question for images that fill the device background… is that a** ‘css background’ **setting? how to center or ‘resize’ the image to get the right size (maximize, cropped, etc) depending on the X device?