add zIndex to theme value

[BenGamma]

At the moment we have multiple zIndex values around the project and it each time a value is added, the dev never know if the new value is going to override an other z-index value, somewhere else in the project.
As suggested by @drydenwilliams we could use a theme value for z-index like :

const zIndex = {
 main: 1,
 header: 2,
 footer: 3
modal: 4
}

[BenGamma]

closed with #736