Add border-gradient mixin

polaris-react/postcss-mixins/border-gradient.js

@@ -0,0 +1,12 @@
+// Applies a border-gradient effect to an element.
+// @param $backgroundColor - Color of element inside the border. Transparent colors are not supported.
+// @param $borderGradientColor - Color gradient to use for the border
+// @param $borderRadius - Polaris BorderRadiusScale custom property.
+module.exports = (_, backgroundColor, borderGradientColor, borderRadius) => ({
+  borderRadius: borderRadius,
+  border: 'solid var(--p-border-width-025) transparent',
+  backgroundColor: backgroundColor,
+  backgroundImage: `linear-gradient(${backgroundColor}, ${backgroundColor}), ${borderGradientColor}`,
+  backgroundOrigin: 'border-box',
+  backgroundClip: 'padding-box, border-box',
+});