Fix ordering of screen breakpoints

Newly add xs breakpoint was not added correctly, causing it to override
the base breakpoints.

tailwind.config.js

@@ -1,3 +1,5 @@
+const defaultTheme = require('tailwindcss/defaultTheme');
+
 module.exports = {
   purge: {
       content: [
@@ -25,10 +27,9 @@ module.exports = {
       '1-1/3': '1 1 33.3333333%',
       '1-1/2': '1 1 50%',
     },
-    extend: {
-      screens: {
-        'xs': '400px',
-      }
+    screens: {
+      'xs': '400px',
+      ...defaultTheme.screens,
     },
   },
   variants: {