By default, the KickstartCassiopeia module allows to set a background-color for your header.
But with one line of css you can make this a gradient. This means one color fades into the other color
Just add the following to your custom css section, and change the colors to your liking:
.container-header {background: linear-gradient(to right, #cc5333, #23074d);}
That's all folks, you now have a gradient in the header!