You could use linear-gradients to achieve this.
Example Here
html, body {
height: 100%;
}
body {
background-image: linear-gradient(90deg,
#F6D6A8 20%,
#F5BA55 20%, #F5BA55 40%,
#F09741 40%, #F09741 60%,
#327AB2 60%, #327AB2 80%,
#3A94F6 80%);
}
Just add vendor prefixes for additional browser support
body {
background: -moz-linear-gradient(90deg, #F6D6A8 20%, #F5BA55 20%, #F5BA55 40%, #F09741 40%, #F09741 60%, #327AB2 60%, #327AB2 80%, #3A94F6 80%);
background: -webkit-linear-gradient(90deg, #F6D6A8 20%, #F5BA55 20%, #F5BA55 40%, #F09741 40%, #F09741 60%, #327AB2 60%, #327AB2 80%, #3A94F6 80%);
background: linear-gradient(90deg, #F6D6A8 20%, #F5BA55 20%, #F5BA55 40%, #F09741 40%, #F09741 60%, #327AB2 60%, #327AB2 80%, #3A94F6 80%);
}
Browser support can be found here.