<style> #home-header3 header { background: rgb(2, 0, 36); background: linear-gradient(90deg, rgba(2, 0, 36, 1) 0%, rgba(9, 9, 121, 1) 35%, rgba(0, 212, 255, 1) 100%); }
</style>
Replace home-header3 with your own header id/name and the color parameters with the ones you need, add the code to Page Settings>Custom Code>Header