It works for me using the following setup, basically just the electron quick start

Run as electron index.js

index.js

const { app, BrowserWindow } = require('electron')
const path = require('path')
const url = require('url')

let win

function createWindow() {
    win = new BrowserWindow({ width: 800, height: 600 })

    win.loadURL(url.format({
        pathname: path.join(__dirname, 'index.html'),
        protocol: 'file:',
        slashes: true
    }))

    win.webContents.openDevTools()

    win.on('closed', () => {
        win = null
    })
}

app.on('ready', createWindow)

app.on('window-all-closed', () => {
    if (process.platform !== 'darwin') {
        app.quit()
    }
})

app.on('activate', () => {
    if (win === null) {
        createWindow()
    }
})

index.html

<webview id="wv1" src="https://www.github.com/" style="display:inline-flex; width:100%; height:140px" nodeintegration></webview>

<script>
    var webview = document.getElementById('wv1');
    webview.addEventListener('dom-ready', function () {
        webview.insertCSS('html,body{ background-color: #FF0000 !important;}')
    });

</script>

Just to be sure you may want to use instead of dom-ready the DOMContentLoaded and instead of insertCss, insertRule.

var webview = document.getElementById('wv1');

webview.addEventListener('DOMContentLoaded', function() {
   webview.insertRule('html,body{ background-color: #FF0000 !important;}',1)
 });

However, if it won't work you may want to try

webview.querySelector(body) !== null

Inside the function.

hope it works.