CSS

rainbow linear gradient css

#grad1 {
  height: 200px;
  background: linear-gradient(45deg, red, orange, yellow, green, blue, indigo, violet, red);
}

gradient rainbow background css

/* Add below code for adding rainbow background in your css element */

#element {
	background:
        linear-gradient(red, transparent),
        linear-gradient(to top left, lime, transparent),
        linear-gradient(to top right, blue, transparent);
	background-blend-mode: screen;
}