Update to Laravel 5.6 is successful！

public/mix-manifest.json

@@ -0,0 +1,4 @@
+{
+    "/js/app.js": "/js/app.js",
+    "/css/app.css": "/css/app.css"
+}

resources/assets/sass/_variables.scss

@@ -5,7 +5,7 @@ $body-bg: #000;
 
 // Typography
 $font-family-sans-serif: "Raleway", sans-serif;
-$font-size-base: 14px;
+$font-size-base: 1rem;
 $line-height-base: 1.6;