Hey Again,

You will have to replace an existing icon with the Twitch code for eg replacing the instagram icon use below css code :

Add this into the Custom CSS box located in your Salient Options panel (All custom css provided is tested in the Live Browser so it will work as intended . If you cant see any changes make sure there is no red cross in the Custom CSS Box as that syntax error would cause all css below it to not show up on the Frontend):

#header-outer #social-in-menu .fa-instagram:before,#header-outer #social-in-menu .fa-instagram:after, .material #slide-out-widget-area.slide-out-from-right .fa-instagram:after {
    content: "\f1e8" !important;
}

Thanks