Hi. Here is code for header color ( gradient) with default values. Change to yours
#sp-header::after {
background-image: -webkit-linear-gradient(137deg,#FF5757 0%,#764AFF 100%);
background-image: -moz-linear-gradient(137deg,#FF5757 0%,#764AFF 100%);
background-image: -o-linear-gradient(137deg,#FF5757 0%,#764AFF 100%);
background-image: linear-gradient(137deg,#FF5757 0%,#764AFF 100%);
}
If you do not want to use the gradient, but you want a solid color, then use such a code
#sp-header::after {
background-image: none !important;
background-color: #000;
}
Make shure that Sticky Header option is ON.
I also want to note that Yoga uses the outdated Helix 3 framework. I would recommend that you use some template based on Helix Ultimate framework.