From your Shopify admin, go to Online Store->Themes.
Find the theme that you want to edit, and then click Customize.
Select one of the gradient options.
If you haven't set up a color gradient yet, several preset options are displayed for you to choose. Select any preset gradients to open the option panel.
If you've already set up a color gradient, then click a gradient option to open the option panel directly.
If you did set up a mono color and want to change to gradient, please remove the color code in CSS code field. Next, click a gradient option to open the option panel directly.