Getting this warning while running ng serve for angular 12 - Unable to locate stylesheet: C:\Workstation\fe\assets\css\core.css

warning message

My angular was 7, but recently I upgraded it to 12.0.5. The ng build goes fine and ng serve is also executing without any error but always getting the above warnings. Also, I am trying to open these css files in browser like localhost:4200/assets/core.css I can see that it's loading css files loading in browser but not sure why this is not applying in the project hence resulting into undesirable styles and design.

my index.html looks like

<link rel="icon" type="image/x-icon" href="assets/images/favicon.ico">
<link rel="stylesheet" type="text/css" href="assets/css/core.css" >
<link rel="stylesheet" type="text/css" href="https://cdn.datatables.net/1.10.25/css/jquery.dataTables.min.css" >
<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.6.2/css/font-awesome.min.css" rel="stylesheet" integrity="sha384-aNUYGqSUL9wG/vP7+cWZ5QOM4gsQou3sBfWRr/8S3R1Lv0rysEmnwsRKMbhiQX/O"  crossorigin="anonymous">
<link rel="stylesheet" type="text/css" href="assets/css/liwid.css">
<link rel="stylesheet" type="text/css" href="assets/css/styleguide.css">
<link rel="stylesheet" type="text/css" href="assets/css/custom.css">

my angular.json

"architect": {
    "build": {
      "builder": "@angular-devkit/build-angular:browser",
      "options": {
        "outputPath": "dist",
        "index": "src/index.html",
        "main": "src/main.ts",
        "polyfills": "src/polyfills.ts",
        "tsConfig": "src/tsconfig.app.json",
        "assets": [
          "src/favicon.ico",
          "src/assets"
        ],
        "styles": [
          "src/styles.css",
          "node_modules/ngx-bootstrap/datepicker/bs-datepicker.css",
          "node_modules/bootstrap/dist/css/bootstrap.css"
        ],
        "scripts": [
          "node_modules/bootstrap/dist/js/bootstrap.js",
          "node_modules/jquery/dist/jquery.slim.js",
          "node_modules/popper.js/dist/umd/popper.min.js"
        ]