[improvement] add constant of border (#4078)

src/sku/components/SkuHeader.tsx

@@ -1,5 +1,6 @@
 import { createNamespace } from '../../utils';
 import { inherit } from '../../utils/functional';
+import { BORDER_BOTTOM } from '../../utils/constant';
 import Icon from '../../icon';
 
 // Types
@@ -45,7 +46,7 @@ function SkuHeader(
   };
 
   return (
-    <div class={[bem(), 'van-hairline--bottom']} {...inherit(ctx)}>
+    <div class={[bem(), BORDER_BOTTOM]} {...inherit(ctx)}>
       <div class={bem('img-wrap')} onClick={previewImage}>
         <img src={goodsImg} />
       </div>