In case you are using node.js (with express)
If you want to serve static files in node.js, you need to use a function. Add the following code to your js file:
app.use(express.static("public"));
Where app is:
const express = require("express");
const app = express();
Then create a folder called public in you project folder. (You could call it something else, this is just good practice but remember to change it from the function as well.)
Then in this file create another folder named css (and/or images file under css if you want to serve static images as well.) then add your css files to this folder.
After you add them change the stylesheet accordingly. For example if it was:
href="cssFileName.css"
and
src="imgName.png"
Make them:
href="css/cssFileName.css"
src="css/images/imgName.png"
That should work