Css files of liberary can not be load
You can follow document


  You may notice the 'theme' key for css which is not present for js. This indicates the style type the css file belongs to.
  
  You can set CSS weights with 5 different levels of styling:



base key assigns a weight of CSS_BASE = -200 
layout key assigns a weight of CSS_LAYOUT = -100 
component key assigns a weight of CSS_COMPONENT = 0; 
state key assigns a weight of CSS_STATE = 100
theme key assigns a weight of CSS_THEME = 200



  This is defined by the SMACSS standard. So here if you specify theme
  it means that the CSS file contains theme related styling which is
  pure look and feel. More info here. You cannot use other keys as these
  will cause strict warnings.


In your case you can use key theme

foundation:
  version: 6.x
  css:
    theme:
      includes/foundation/css/app.css: {}
      includes/foundation/css/foundation.css: {}
      includes/foundation/css/foundation.min.css: {}
  js:
    includes/foundation/js/app.js: {}
    includes/foundation/js/foundation.js: {}
    includes/foundation/js/foundation.min.js: {}
    includes/foundation/js/jquery.js: {}
    includes/foundation/js/what-input.js: {}
  dependencies:
    - core/drupal
    - core/jquery
    - core/jquery.once