Add custom css in drupal 8

In your theme_name.info.yml add below code

libraries:
  - 'theme_name/theme'

theme - custom name

open theme_name.libraries.yml file and add below code

theme:
  js:
    js/jquery-1.11.3.min.js: {}
    js/bootstrap.min.js: {}
    js/jquery.fullPage.js: {}
    js/main.js: {}
  css:
    theme:
      css/bootstrap.min.css: {}
      css/font-awesome.min.css: {}
      css/jquery.fullPage.css: {}
      css/style.css: {}
      css/media.css: {}  
  dependencies:
   

Notes

Don't add tab space on .yml file 
add normal space bar space