title: Argument `$color` of `darken($color, $amount)` must be a color Bootstrap 4 Angular 6
body: <p>Attempting to use Bootstrap 4 a la carte in my Angular 6 app like so in my angular.json file...</p>
  
  <pre><code>"styles": [
                "node_modules/bootstrap/scss/_functions.scss",
                "node_modules/bootstrap/scss/_variables.scss",
                "node_modules/bootstrap/scss/_mixins.scss",
                "src/styles.scss"
              ] 
  </code></pre>
  
  <p>Which yields this error in the terminal after sass load attempt is made...</p>
  
  <blockquote>
    <p>ERROR in ./node_modules/bootstrap/scss/_variables.scss
    (./node_modules/raw-loader!./node_modules/postcss-loader/lib??embedded!./node_modules/sass-loader/lib/loader.js??ref--15-3!./node_modules/bootstrap/scss/_variables.scss)
    Module build failed (from ./node_modules/sass-loader/lib/loader.js):</p>
    
    <p>undefined
                               ^
          Argument <code>$color</code> of <code>darken($color, $amount)</code> must be a color</p>
  </blockquote>
  
  <p>This is the line in question in _variables.scss: 
  <code>$link-hover-color: darken($link-color, 15%) !default;</code></p>
  
  <p>Two lines above it's declared: 
  <code>$link-color: theme-color("primary") !default;</code></p>
  
  <p>I've looked through many possible solutions on here but most of them say to put functions before vars which is already done yet the error still persists.</p>
  
  <p>Any help greatly appreciated.</p>