0. position-shorthand.styl

position(
  $position,
  $top = null,
  $right = $top,
  $bottom = $top,
  $left = $right
)
  position: $position
  top: $top
  right: $right
  bottom: $bottom
  left: $left

1. c-modal.styl

@import 'position-shorthand.styl'

.c-modal
  body.c-modal-active > &
    display: block

  &
    display none
    position: fixed 0

  > .panel
    position: fixed 50% 50% initial initial
    
  > .background
    position: relative 50% 25%
    background-color: rgba(#000, .75)

2. c-modal.css

body.c-modal-active > .c-modal {
  display: block;
}
.c-modal {
  display: none;
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
}
.c-modal > .panel {
  position: fixed;
  top: 50%;
  right: 50%;
  bottom: initial;
  left: initial;
}
.c-modal > .background {
  position: relative;
  top: 50%;
  right: 25%;
  bottom: 50%;
  left: 25%;
  background-color: rgba(0,0,0,0.75);
}