Z-Index

The z-index property specifies the stack order of an element. An element with greater stack order is always in front of an element with a lower stack order.

Class	Properties
`z-auto`	`z-index: auto;`
`z--1`	`z-index: -1;`
`z-0`	`z-index: 0;`
`z-1`	`z-index: 1;`
`z-10`	`z-index: 10;`
`z-20`	`z-index: 20;`
`z-30`	`z-index: 30;`
`z-40`	`z-index: 40;`
`z-50`	`z-index: 50;`
`z-99`	`z-index: 99;`

'z-index': (
    'enabled'    : true, // true or false
    'property'   : 'z-index', // css property name (don't change it !)
    'prefix'     : 'z-', // prefix of class name
    'specificity': !important, // or null
    'unit'       : null, // length/angle units (px, rem, em, %, vh, vw etc)
    'args'       : null, // for extra variables
    'variables'  : (auto,-1,0,1,10,20,30,40,50,99)
)