title: How to import @material/theme and @material/textfield style to create own theme package
body: <p>I'm trying to create a custom <code>@material/theme</code> package containing custom colors and only components used(<code>@material/textfield/mdc-text-field</code> in this example), following <a href="https://material.io/develop/web/components/theme/" rel="nofollow noreferrer">this documentation</a> </p>
  
  <h2>@mycorp/scss</h2>
  
  <p><strong>variables.scss</strong></p>
  
  <pre><code>$primary-color: #ff742a;
  $secondary-color: #ffd42a;
  $text-color: #383838;
  $background-color: #F6F6F6;
  </code></pre>
  
  <p><strong>material.scss</strong></p>
  
  <pre><code>@import './variables.scss';
  
  // Update Theme
  @use "@material/theme" with (
    $primary: $primary-color,
    $secondary: $secondary-color,
    $background: $background-color,
  );
  
  // Import only Text Field
  @use '@material/textfield/mdc-text-field';
  </code></pre>
  
  <p><strong>index.scss</strong></p>
  
  <pre><code>@import './variables.scss';
  @import './material.scss';
  </code></pre>
  
  <h2>Project</h2>
  
  <p>I'm using React and Webkit + Babel to run the project, and I installed <code>@rmwc/textfield</code> to generate the HTML.</p>
  
  <p>I import my style on my <code>app.scss</code> by importing the index of the package:  </p>
  
  <p><code>@import "@mycorps/scss/src/index.scss";</code> </p>
  
  <p>I know that everything else is working fine (variables, ..) but <strong>No Style for Material Design</strong>.</p>
  
  <h2>Another try</h2>
  
  <p>I Also tried to replace <code>@use</code> by <code>@import</code> in order to "write" the text-field theme (i guess ?) but I got the error when I try to build it:</p>
  
  <pre><code>ERROR in ./src/style/app.scss (./node_modules/css-loader/dist/cjs.js!./node_modules/postcss-loader/src??ref--6-2!./node_modules/sass-loader/dist/cjs.js??ref--6-3!./src/style/app.scss)
  Module build failed (from ./node_modules/sass-loader/dist/cjs.js):
  SassError: Invalid CSS after "@include mixins": expected 1 selector or at-rule, was ".core-styles;"
          on line 30 of ../../../dev/mycorps/packages/mycorps-scss/node_modules/@material/textfield/mdc-text-field.scss
          [...]
  &gt;&gt; @include mixins.core-styles;
  
     ---------^
  
   @ ./src/style/app.scss 2:26-201
   @ ./src/index.js
  
  </code></pre>
  
  <p>Same result when I tried both solutions without results:</p>
  
  <ul>
  <li><a href="https://github.com/material-components/material-components-web/issues/351#issuecomment-298796798" rel="nofollow noreferrer">https://github.com/material-components/material-components-web/issues/351#issuecomment-298796798</a></li>
  <li><a href="https://github.com/webpack-contrib/sass-loader/issues/804#issuecomment-586095020" rel="nofollow noreferrer">https://github.com/webpack-contrib/sass-loader/issues/804#issuecomment-586095020</a></li>
  </ul>