Use z-index utility classes to control the index of your elements in the z dimension. When two elements overlap each other, z-index establishes what element overlaps the other. Usually elements with larger z-index overlap elements with smaller ones.
Class name  | z-index value  | Used in  | 
|---|---|---|
-z--0 | 0 | Initial value | 
-z--1 | 1 | Prominent elements (E.g. focused buttons) | 
-z--10 | 10 | Dropdowns | 
-z--20 | 20 | Fixed elements (E.g. drawer) | 
-z--30 | 30 | Backdrops | 
-z--40 | 40 | Fixed elements with backdrop (E.g. drawer with backdrop) | 
-z--50 | 50 | Modals | 
-z--60 | 60 | Popovers | 
-z--70 | 70 | Tooltips | 
-z--80 | 80 | - | 
-z--90 | 90 | - | 
-z--100 | 100 | - | 
-z--auto | auto | - | 
<div class="-position--relative">
  <div class="-position--absolute -z--0  -bg--info">-z--0</div>
  <div class="-position--absolute -z--10 -bg--danger">-z--10</div>
  <div class="-position--absolute -z--20 -bg--warning">-z--20</div>
  <div class="-position--absolute -z--30 -bg--success">-z--30</div>
  <div class="-position--absolute -z--40 -bg--green-40">-z--40</div>
  <div class="-position--absolute -z--50 -bg--grey-20">-z--50</div>
  <div class="-position--absolute -z--60 -bg--grey-10">-z--60</div>
  <div class="-position--absolute -z--70 -bg--black">-z--70</div>
  <div class="-position--absolute -z--80 -bg--brand">-z--80</div>
</div>