Remove 1-level import from '@material-ui/core'

Author: dmtrKovalenko

docs/pages/demo/datepicker/CustomInput.example.jsx

@@ -1,5 +1,5 @@
 import React, { useState } from 'react';
-import { styled } from '@material-ui/core';
+import { styled } from '@material-ui/core/styles';
 import { DesktopDatePicker } from '@material-ui/pickers';
 
 const InputContainer = styled('div')({