Step 1:
Access your Shopify Admin's Theme Library to open the theme customizer.
Step 2:
In the theme customizer, go to the Theme settings tab and at the bottom of the Theme settings panel, you'll see an option for Custom CSS.
Step 3
Copy and paste the following CSS code into the Custom CSS field:
.product-item--sold-out .badge { background-color: red; color: green; }
Step 4:
Replace red and green with your desired colors. The background-color property controls the background color of the sold-out badge, and the color property controls the text color.
Step 5:
Save Your Changes