How to include custom styles with Angular CLI

Simplest way is to create a folder (in src/) call it styles/ place your style.css (or SCSS and all other _variables.scss files) in it then amend angular-cli.json thus:

"styles": [
  "styles/styles.css"
]

As per your folder structure, correct path is assets/css/styles.css so change it in your angular-cli.json file as below and check it.

"styles": [
  "styles.css",
  "assets/css/styles.css"
],