compileStyles(addonStylesTree) {
    if (addonStylesTree) {
      let preprocessedStylesTree = this._addonPreprocessTree('css', addonStylesTree);

      let processedStylesTree = preprocessCss(preprocessedStylesTree, '/', '/', {
        outputPaths: { addon: `${this.name}.css` },
        registry: this.registry,
        treeType: 'addon-styles',
      });
      processedStylesTree = new Funnel(processedStylesTree, {
        destDir: `${this.name}/__COMPILED_STYLES__`,
      });

      return this._addonPostprocessTree('css', processedStylesTree);
    }
  }
Runs the styles tree through preprocessors.

    @private
    @method compileStyles
    @param {Tree} addonStylesTree Styles file tree
    @return {Tree} Compiled styles tree