next.js/examples/with-ant-design-less/next.config.js

/* eslint-disable */
const withLess = require('@zeit/next-less')
const lessToJS = require('less-vars-to-js')
const fs = require('fs')
const path = require('path')

// Where your antd-custom.less file lives
const themeVariables = lessToJS(
  fs.readFileSync(path.resolve(__dirname, './assets/antd-custom.less'), 'utf8')
)

// fix: prevents error when .less files are required by node
if (typeof require !== 'undefined') {
  require.extensions['.less'] = file => {}
}

module.exports = withLess({
  lessLoaderOptions: {
    javascriptEnabled: true,
    modifyVars: themeVariables // make your antd custom effective
  }
})