[sass] support imports that start with ~

.postcssrc.js

@@ -2,7 +2,15 @@
 
 module.exports = {
   "plugins": {
-    "postcss-import": {},
+    "postcss-import": {
+      resolve(id) {
+        // sass imports that start with ~ (which refers node_modules) should be supported
+        if (id.startsWith('~')) {
+          return id.slice(1);
+        }
+        return id;
+      }
+    },
     "postcss-url": {},
     // to edit target browsers: use "browserslist" field in package.json
     "autoprefixer": {}