Sliding Diagonals Background Effect

index.html

<div class="bg"></div>
<div class="bg bg2"></div>
<div class="bg bg3"></div>
<div class="content">
  <h1>Sliding Diagonals Background Effect</h1>
</div>

sliding-diagonals-background-effect.markdown

Sliding Diagonals Background Effect

An animated background under the content.

A Pen by Chris Smith on CodePen.

License.

style.css

html {
  height:100%;
}

body {
  margin:0;
}

.bg {
  animation:slide 3s ease-in-out infinite alternate;
  background-image: linear-gradient(-60deg, #6c3 50%, #09f 50%);
  bottom:0;
  left:-50%;
  opacity:.5;
  position:fixed;
  right:-50%;
  top:0;
  z-index:-1;
}

.bg2 {
  animation-direction:alternate-reverse;
  animation-duration:4s;
}

.bg3 {
  animation-duration:5s;
}

.content {
  background-color:rgba(255,255,255,.8);
  border-radius:.25em;
  box-shadow:0 0 .25em rgba(0,0,0,.25);
  box-sizing:border-box;
  left:50%;
  padding:10vmin;
  position:fixed;
  text-align:center;
  top:50%;
  transform:translate(-50%, -50%);
}

h1 {
  font-family:monospace;
}

@keyframes slide {
  0% {
    transform:translateX(-25%);
  }
  100% {
    transform:translateX(25%);
  }
}

Hello!
How it could be changed with the flex help in order to bind animation not to the whole background, but to .content box only?