using the express.static middleware in an Express app.

Serve static content for the app from the “public” directory in the application directory:

index.js
// GET /style.css etc
app.use(express.static(path.join(__dirname, 'public')))

在工程目录下新建一个public文件夹，该文件夹下放static contents, like app.css, app.js, etc.

app.css

body{
    background-color: pink;
}

在对应的ejs文件下添加依赖： 

<link rel="stylesheet" href="/app.css">