html, body {
  width: 100%;
  height: 100%;
  margin: 0px;
  padding: 0px;
  border: 0px;
  overflow: hidden;
}
.modal-window {
  position: fixed;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  background-color: rgba(0,0,0,0.4);
}
.modal-window > * {
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
}
.horizontal {
  display: flex;
  flex-flow: row;
}
.stretch {
  flex: 1;
}
.vertical {
  display: flex;
  flex-flow: column;
}
.align-start {
  align-items: flex-start;
}
.centered {
  align-items: center;
}
.align-end {
  align-items: flex-end;
}