Add to Cart Button uses Primary Button Style, so you can change it in Site Styles or use Custom CSS.
A. Use Site Styles
#1. Click the Site Styles icon
#2. Click Colors
#3. Choose a theme color
In my example, it is LIGHTEST 1 . To find this, just open a Product in edit mode > Click Site Styles icon > Colors > You will see the theme name.
#4. Scroll down to this
B. Use CSS
Just use this code to Custom CSS box
div.sqs-add-to-cart-button-inner {
color: #f1f !important;
}
div.sqs-add-to-cart-button {
background-color: #ff0 !important;
}
Result