fix: don't override sassLoader data

Author: kevinmarrec

lib/module.js

@@ -34,9 +34,15 @@ module.exports = function (moduleOptions) {
 
     // Customize sass-loader options
     Object.assign(this.options.build.loaders.scss, sassLoaderOptions)
-    sassLoaderOptions.data = options.customVariables.map(path => `@import '${path}'`).join('\n')
     Object.assign(this.options.build.loaders.sass, sassLoaderOptions)
 
+    // Custom variables
+    const sassLoaderData = this.options.build.loaders.sass.data
+    if (options.customVariables.length > 0 && typeof sassLoaderData !== 'function') {
+      const imports = options.customVariables.map(path => `@import '${path}'`).join('\n')
+      this.options.build.loaders.sass.data = sassLoaderData ? sassLoaderData.concat('\n', imports) : imports
+    }
+
     // Add styles
     if (options.treeShake) {
       this.options.css.push('vuetify/src/styles/main.sass')