Syntax:

background-image: linear-gradient(direction, color-stop1, color-stop2, ...);

For a 3 stop color gradient:

The code is

background: rgb(246,178,16);
background: linear-gradient(135deg, rgba(246,178,16,1) 0%, rgba(21,21,227,0.7) 70%, rgba(0,212,255,1) 100%);

For max compatibility:

background: rgb(246,178,16);
background: -moz-linear-gradient(135deg, rgba(246,178,16,1) 0%, rgba(21,21,227,0.7) 70%, rgba(0,212,255,1) 100%);
background: -webkit-linear-gradient(135deg, rgba(246,178,16,1) 0%, rgba(21,21,227,0.7) 70%, rgba(0,212,255,1) 100%);
background: linear-gradient(135deg, rgba(246,178,16,1) 0%, rgba(21,21,227,0.7) 70%, rgba(0,212,255,1) 100%);
filter: progid:DXImageTransform.Microsoft.gradient(startColorstr="#f6b210",endColorstr="#00d4ff",GradientType=1);

 

Ref

https://cssgradient.io/