Undefined variable: "$linkColor"の回避

###前提・実現したいこと
rails5.1.1でWYSIWYGのMedium Editorの導入後の画像をS３へアップロード
下のサイトを参考に作成
MediumライクなWYSIWIGエディターを簡単にRailsで実装する

###発生している問題・エラーメッセージ
Undefined variable: "$linkColor".

```![イメージ説明](c959d4bb9a607e70e2054a68556c5429.png)
多分bootstrapないで使われている変数？


###application.js

###// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below.
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, or any plugin's
// vendor/assets/javascripts directory can be referenced here using a relative path.
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// compiled file. JavaScript code in this file should be added after the last require_* statement.
//
// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
// about supported directives.
//

//= require jquery
//= require jquery_ujs
//= require jquery/dist/jquery.min.js
//= require medium-editor/dist/js/medium-editor
//= require handlebars/handlebars.runtime.min.js
//= require jquery-sortable/source/js/jquery-sortable-min.js
//= require blueimp-file-upload/js/vendor/jquery.ui.widget.js
//= require blueimp-file-upload/js/jquery.iframe-transport.js
//= require blueimp-file-upload/js/jquery.fileupload.js
//= require medium-editor-insert-plugin/dist/js/medium-editor-insert-plugin.min.js
//= require medium-editor
//= require rails-ujs
//= require turbolinks
//= require_tree .
//= require froala_editor.min.js
//= require plugins/align.min.js
//= require plugins/char_counter.min.js
//= require plugins/code_beautifier.min.js
//= require plugins/code_view.min.js
//= require plugins/colors.min.js
//= require plugins/emoticons.min.js
//= require plugins/entities.min.js
//= require plugins/file.min.js
//= require plugins/font_family.min.js
//= require plugins/font_size.min.js
//= require plugins/fullscreen.min.js
//= require plugins/image.min.js
//= require plugins/image_manager.min.js
//= require plugins/inline_style.min.js
//= require plugins/line_breaker.min.js
//= require plugins/link.min.js
//= require plugins/lists.min.js
//= require plugins/paragraph_format.min.js
//= require plugins/paragraph_style.min.js
//= require plugins/quick_insert.min.js
//= require plugins/quote.min.js
//= require plugins/save.min.js
//= require plugins/table.min.js
//= require plugins/url.min.js
//= require plugins/video.min.js
//= require languages/ja.js

$(function () {
$(".editable").mediumInsert({
editor: editor,
addons: {
images: {
fileUploadOptions: {
url: 'upload_image',
acceptFileTypes: /(.|/)(gif|jpe?g|png)$/i
}
}
}
});
});


###application.css
```/*
 * This is a manifest file that'll be compiled into application.css, which will include all the files
 * listed below.
 *
 * Any CSS and SCSS file within this directory, lib/assets/stylesheets, or any plugin's
 * vendor/assets/stylesheets directory can be referenced here using a relative path.
 *
 * You're free to add application-wide styles to this file and they'll appear at the bottom of the
 * compiled file so the styles you add here take precedence over styles defined in any other CSS/SCSS
 * files in this directory. Styles in this file should be added after the last require_* statement.
 * It is generally better to create a new file per style scope.
 *
 *= require_tree .
 *= require_self
 *= require froala_editor.min.css
 *= require froala_style.min.css

 *= require plugins/char_counter.min.css
 *= require plugins/code_view.min.css
 *= require plugins/colors.min.css
 *= require plugins/emoticons.min.css
 *= require plugins/file.min.css
 *= require plugins/fullscreen.min.css
 *= require plugins/image_manager.min.css
 *= require plugins/image.min.css
 *= require plugins/line_breaker.min.css
 *= require plugins/quick_insert.min.css
 *= require plugins/table.min.css
 *= require plugins/video.min.css
 *= require font-awesome
 *= require medium-editor/themes/flat
 *= require medium-editor/dist/css/medium-editor
*= require medium-editor/dist/css/themes/flat
*= require medium-editor-insert-plugin/dist/css/medium-editor-insert-plugin
*= require medium-editor-insert-plugin/dist/css/medium-editor-insert-plugin-frontend
 */
/*@import "bootstrap-sprockets";*/
/*@import "bootstrap";*/
@import "font-awesome-sprockets";
@import "font-awesome";
@charset "UTF-8";
@import url(http://fonts.googleapis.com/earlyaccess/notosansjapanese.css);
html{
  overflow-y:scroll;
}

body, h1, h2, h3, h4, h5, p, ul, ol, li, dl, dt, dd{
  margin:0px;
  padding:0px;
}

body{
  background: #fff;
  text-align:center;
  font-size:16px;
  line-height:1.4;
  min-width:100%;
  -webkit-text-size-adjust:100%;
}

 *= require medium-editor/dist/css/themes/flata{
  outline:none;
  color:inherit;
  text-decoration:none;
}

img{
  border:0px;
  vertical-align:bottom;
}

body {
  font-family: "Noto Sans Japanese", "ヒラギノ角ゴ Pro W3","Hiragino Kaku Gothic Pro", "ヒラギノ角ゴ ProN W3","Hiragino Kaku Gothic ProN", "游ゴシック",YuGothic, "メイリオ","Meiryo", "HGPｺﾞｼｯｸM", "Osaka","ＭＳ Ｐゴシック",sans-serif,Century Gothic,Arial;
}

li{
  list-style:none;
}

body{
    font-size: 16px;
    font-family: "Graphik Meetup",helvetica,arial,sans-serif;
    font-weight: 400;
}

.clear{
  clear:left;
}

.container-fluid{
  padding-left: 0;
  padding-right: 0;
}


/*ここからheader*/
#header{
  width: 100%;
  min-height: 60px;
}

#header-wrap{
  max-width: 1200px;
  margin-left: 40px;
  margin-right: 40px;
}

#header-left{
  float:left;
  width: 20%;
}
#header-right{
  float: left;
  width: 40%;
}

#header-right ul li{
  float: left;
  margin-left: 35px;
  line-height: 60px;
}

a .logo :hover{
    background-color: transparent;
}
/*headerはここまで*/


/*ここからcontents*/
/*contentsはここまで*/

/*ここからfooter*/
.upper-footer{
   background-color: #a8cee0;
}

.lower-footer{
  background-color: #033850;
}

.lower-footer ul li{
  color: #8d8e92;
  line-height:30px;

}
/*footerはここまで*/

初めての質問でわかりにくいと思いますがお知恵をお貸しください
よろしくお願いいたします